Plant Morphology: The Historic Concepts of Wilhelm Troll, Walter Zimmermann and Agnes Arber
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Annals of Botany 88: 1153-1172, 2001
doi:10.1006/anbo.2001.1544, available online at http://www.idealibrary.com on
Plant Morphology: The Historic Concepts of Wilhelm Troll, Walter Zimmermann
and Agnes Arber
REGINE CLABEN-BOCKHOFF*
Institut fur Spezielle Botanik und Botanischer Garten, Johannes Gutenberg-Universitd't, Bentzelweg 2,
D-55099 Mainz, Germany
Received: 11 May 2001 Returned for revision: 26 June 2001 Accepted: 14 August 2001
Recent molecular systematic and developmental genetic findings have drawn attention to plant morphology as a
discipline dealing with the phenotypic appearance of plant forms. However, since different terms and conceptual
frameworks have evolved over a period of more than 200 years, it is reasonable to survey the history of plant
morphology; this is the first of two papers with this aim. The present paper deals with the historic concepts of Troll,
Zimmermann and Arber, which are based on Goethe's morphology. Included are contrasting views of 'unity and
diversity', 'position and process', and 'morphology and phylogeny', which, in part, are basic views of current plant
morphology, phylogenetic systematics and developmental genetics. Wilhelm Troll established the 'type concept' and
the 'principle of variable proportions'. He has provided the most comprehensive overview of the positional relations
of plant forms. Agnes Arber started from the universal dynamics of life and attempted to describe all structures as
processes. She paid attention to 'repetitive branching', 'differential growth', and 'parallelism'. As a result she has
recently been rediscovered by developmental botanists. Walter Zimmermann rejected any metaphysical influence on
plant form and instead called for objective procedures. He was mainly interested in phylogenetic 'character
transformation' and the 'reconstruction of genealogical lines'. Guided by the example of flower-like inflorescences, a
future paper will deal with functional and developmental constraints influencing plant forms. Recent morphological
concepts ('trialectical', 'continuum'/'fuzzy', 'process morphology') will be discussed and related to current
morphological and developmental genetic research. © 2001 Annals of Botany Company
Key words: Plant form, plant morphology, natural philosophy, homology, phylogeny, Goethe, Troll, Arber,
Zimmermann, typology, character transformation, differential growth, complementarity.
INTRODUCTION findings require a reconsideration of the meaning, the con-
tent and the conceptual framework of plant morphology.
Plant morphology deals with the phenotypic appearance or
Since its introduction by Goethe in the late 18lh century,
'form' of plants. However, what is the form of a plant?
modern plant morphology has had an eventful history.
Does it exist at all? Are our concepts adequate to explain
Many terms e.g. 'morphology', 'homology', and 'type'
what we call a plant form?
originate from pre-Darwinian times and are used today in
These questions are very old and were discussed in Greek
different contexts. Goethe (1790) realised that the diversity
antiquity and the Middle Ages. They have been answered
of forms might be reduced to a number of 'archetypes'. To
either in the context of Aristotle's metaphysics or the
him, these archetypes represented abstract conclusions from
Christian belief in Creation, always according to the spirit
his empirical observations [see Goethe's famous conversa-
of the times (see Arber, 1950). Even today, during the age
tion with Schiller in Kuhn (1987 pp. 434-438)], but in the
of evolutionary and molecular genetics, these questions are
age of phylogenetic systematics they became transformed
still pressing.
into ancestral types (see Arber, 1946). This confusion existed
for a long time and might even persist today. For this reason,
the relationship between morphological typology and
Two hundred years of plant morphology phylogenetic grouping is re-examined in the present paper.
Our knowledge of plants has increased dramatically in the In Germany, discussion of typology and phylogeny is
last decades. Molecular techniques have promoted studies in closely related to the names of Wilhelm Troll and Walter
phylogenetic systematics and developmental genetics. Plant Zimmermann. Their bitter and often polemic controversy is
forms have been analysed under the functional view of well documented, and illustrates their conflicting views (see
biomechanics and pollination ecology. The need to care for Nickel, 1996 pp. 58-60). Troll (1925,1928) followed Goethe
the world's natural heritage has enhanced studies in bio- in taking the plant types to be ideas while Zimmermann
diversity. All these disciplines are either based on plant (1930, 1937) only accepted types as natural groups having
morphology or are closely related to it. Each of them deals evolved from common ancestors. At the same time, Agnes
with a different aspect of plant form. Diverse views and new Arber (1950) published The natural philosophy ofplant form.
She came from a perspective focused on the open growth of
* E-mail classenbfojmail.uni-mainz.de plants and the continuous change of plant forms during
0305-7364/01/121153 + 20 S35.00/00 % 2001 Annals of Botany Company1154 Clafien-Bockhoff—Plant Morphology
ontogeny. In having morphogenetic change in mind, she plant morphology, phylogenetic systematics and develop-
added a third aspect to the debate on morphology. mental biology (see Fig. 1). It is not only interesting to see
how concepts have evolved and changed through the ages
but it is also necessary to remember their historical roots (see
The historic concepts of Troll, Zimmermann and Arber
Arber, 1950). Many misunderstandings and much confusion
(Fig. 1) of terms have originated in historic disputes and interpreta-
Agnes Arber (1879-1960), Walter Zimmermann (1892- tions. For this reason the present paper summarizes the
1980) and Wilhelm Troll (1897-1978) were born in the last three diverse historical concepts.
quarter of the 19th century and died at the ages of 81, 88 and I do not attempt to summarize the entire work of the
81, respectively. They were enthusiastic botanists, doing an authors concerned; this would be beyond the scope of the
immense amount of empirical work and writing important present paper and, moreover, would require support by
books (see selected references). However, although they were historians and philosophers of science. I focus predomi-
contemporaries, they followed completely different concepts nantly on the controversial views concerning the antitheses
and philosophies (Troll, 1925, 1928, 1951; Zimmermann, of'unity and diversity', 'statics and dynamics' and 'idealism
1930, 1931, 1937; Arber, 1946, 1950; Hauke, 1996; Nickel, and rationalism'. I comment briefly on Goethe's
1996). Troll and Arber were influenced by Goethe and morphology before beginning the individual views because
followed metaphysical ideas by seeking the truth behind all three authors refer to Goethe's work. I endeavour to
facts. In contrast, Zimmermann rejected any metaphysical show that the different concepts are the result of different
influence on science. He was predominantly interested in intentions, procedures and philosophies. Bearing this in
phylogeny and in the evolutionary history of form. Troll and mind we can learn from history that even objective research
Zimmermann both intended to reduce the diversity of living includes subjective elements and that different views can
forms and thus agreed in accepting types and groupings, complement each other rather than being antithetic (see
while Arber focused on dynamic processes and thus could Rutishauser and Sattler, 1985).
not accept static types for fundamental reasons. Zimmer-
mann and Arber included morphology in biology, under-
standing it—in Arber's terminology—as a 'partial view' of ARCHETYPES AND METAMORPHOSIS:
the 'whole'. Troll, however, separated morphology from all GOETHE'S MORPHOLOGY
analytical approaches and thus isolated German morph- Johann Wolfgang von Goethe (1749-1832) was not only a
ology for many years. famous poet but also the founder of 'morphology'. For 200
years he has been honoured as one of the most outstanding
and versatile men of his age and a great deal has been
The aim of the present paper written about his work. The present summary of Goethe's
The present paper is the first of two papers dealing with fundamental influence on plant morphology is based
plant morphology. It summarizes selected historical views of mainly on the comments given by Steiner (1883), Troll
plant form and re-examines the meaning of'morphology'. A (1928), Arber (1946), Froebe (1986) and Kuhn (1987, 1992).
second paper is planned which will consider modern
concepts including functional and genetic constraints on
plant form. Plant morphology
The concepts of Troll, Zimmermann and Arber are histo- Goethe's interest in plants began with an ambition to
rical views of plant form. As such they are part of current identify the species around him. At first he was much
philosophy
FIG. 1. The different approaches to the plant form of Arber, Troll and Zimmermann (see text).Clafien-Bockhoff—Plant Morphology 155
impressed by the systematic studies of Linne, but then The archetypal plant ("UrpfianzeV
rejected his analytical approach. This rejection originated in
him, as Goethe was a thoroughly spiritual and sensitive Goethe admired nature for his entire life. He was fasci-
person whose philosophy of life was synthetic and holistic. nated by the diversity of forms and their dynamic changes
and he often expressed his deep feelings in poems and letters.
According to Goethe, analytical studies inevitably have
But he was also irritated by this diversity which apparently
to remain insufficient because living organisms are only
contradicted his holistic view. He became seriously inter-
complete as entities in themselves and not as fragments. To
ested in plants and tried to understand what unity might be
understand the entire organism it is imperative to also
behind the diversity. Goethe started to observe plants in
approach it from the synthetic point of view. Goethe intro-
detail. During his studies on seeds and seedlings in 1775/76,
duced 'morphology' as an approach to both inorganic
the idea arose that there must be some sort of 'archetypal
and organic entities. Correspondingly, plant morphology
plant' which all living plants might refer to. Goethe (1786/
deals with the outer appearance of the entire organism
87) vividly described how he found this 'Urpftanze' or
(see Fig. 2).
'archetypal plant' during his 'Italian journey'. Although he
In the present context two aspects of Goethe's view of had first expected it to be a real plant (17.4.1787), he
plant morphology are particularly interesting: (1) Goethe's suddenly recognized that it was a pure idea of a plant.
central idea was that a dynamic force was inherent in all Goethe was overwhelmed by his finding and in particular by
organisms. He concluded that this dynamic process was life the simplicity of the underlying principle 'everything is leaf
itself and therefore that all living organisms changed con- (see letter to Charlotte von Stein, June 1787).
tinuously. Thus, Goethe took plant form to be a changing
unit and correspondingly asked morphology to study both Goethe (1790) summarized his views in his famous
the form and its change (see Fig. 2). (2) Goethe (see Malsch, botanical work Versuch die Metamorphose der Pflanzen zu
1959 p. 657) took morphology as a purely synthetic disci- erkldren (An attempt to interpret the metamorphosis of
pline instead of an analytical one. In this respect he con- plants; translated by Arber, 1946). The simple answer to his
trasted it with physiology, not as an opposite but as a question concerning unity and diversity was given in the
complement. Both disciplines agree in requiring botanists to conclusion that the 'leaf was the primary plant organ that
disregard their personal feelings and to concentrate on the underwent successive changes ['Alles ist Blatt und durch
'object under observation' (Goethe, 1793). However, diese Einfachheit wird die grofite Mannigfaltigkeit moglich';
empirical results remain incomplete as long as they are not Goethe in Kuhn (1987 p. 84)]. Goethe was, in fact, the first
combined with intellectual conclusions. Thus, Goethe's to state clearly that cotyledons, foliage leaves and floral
thought is characterized by the holistic approach of unifying organs were identical in being 'leaves'.
objective sense and subjective perception. Goethe did not use the term 'leaf as a foliage leaf, but as
an idea, which is realised by diverse manifestations (see
Arber, 1946 p. 81). Consequently, he applied the term to all
lateral appendages of the growing apex in a similar manner
Morphologie for cotyledons up to carpels and vice versa but he took the
(Johann Wolfgang von Goethe 1807) 'leaf itself to be immaterial (Fig. 4).
Ruht auf der Uberzeugung, dafi alles was Metamorphosis
sei sich auch andeuten und zeigen miisse.
Von den ersten physischen und chemischen The primary unit of the plant form including the leaf itself,
the internode below and the bud in its axil is repeated and
Elementen an, bis zur geistigen Aufierung
des Menschen lassen wir diesen Grundsatz
gelten.
Wir wenden uns gleich zu dem was Gestalt
hat. Das Unorganische, das Vegetative, das
Animale, das Menschliche deutet sich alles
selbst an, es erscheint als was es ist unserm
dufiern unserm inneren Sinn.
Die Gestalt ist ein Bewegliches, ein
Werdendes, ein Vergehendes. Gestaltenlehre
ist Verwandlungslehre. Die Lehre der
Metamorphose ist der Schliissel zu alien
Zeichen der Natur.
FIG. 2. Goethe's explanation of 'morphology' (Kuhn, 1987 p. 349). Fid. 3. Goethe's original illustration of 'the node with the leaf, the
Note his comprehensive and holistic approach to morphology and his 'sequence of nodes' and the transition toflowering(from left to right)
dynamic view of form. (after Schneckenburger, 1998).1156 ClaPen-Bockhoff—Plant Morphology
continuously changed during the growth of a plant (Fig. 3).
Goethe (1790) called this process of change 'metamorpho-
sis'. He assumed that plant saps regulated the growth
processes, which usually included three steps of metamor-
phosis: from seed to foliage; from bracts to flowers; and
from carpels to fruits (Fig. 6A). The diversity of lateral
appendages thus depends on metamorphosis which at all
stages realises the underlying 'leaf in a different form.
Goethe adopted the term 'metamorphosis' from ancient
mythology in order to illustrate that all living forms are
unstable and changeable (see Fig. 2). Only the 'type behind
diversity' is fixed and permanent. Thus, in Goethe's view,
metamorphosis is the process that realises diverse forms of
an immaterial idea ('... the process, by which one and the
same organ presents itself to our eyes under protean forms,
has been called the Metamorphosis of Plants'; Goethe, 1790:
§ 4, translated by Arber, 1946 p. 91).
Goethe's use of the term metamorphosis differs from
present usage which is rather restricted to the leaf sequence
(Fig. 4). Nowadays, metamorphosis describes the gradual
change of the foliage with age and thus might imply that
carpels, for example, are taken to be modified leaves (Troll,
1937 p. 36). In Goethe's view, however, all forms are
manifestations of one and the same underlying type. They
merely illustrate the dynamic force of metamorphosis. Arber
(1937 p. 173, 1946 p. 75) recognized this aspect of Goethe's
view and preferred the term 'phyllome' to the term 'leaf to
indicate the equality in principle of all lateral appendages.
Goethe's metaphysical view of nature
Goethe's studies on plant morphology were embedded in
his metaphysical view that nature was the 'personification of
the universe' (e.g. Goethe, 1828): it is unity and ever Fio. 4. Different views of leaf metamorphosis. The leaf sequence of
changing diversity at the same time (Fig. 5). Hellehorus foetklus (Leistikow and Kockel, 1990) illustrates the usual
application of the term 'metamorphosis' to the longitudinal and
Unity exists because all plant forms are manifestations of unidirectional series of leaves (double arrow). This view was shared by
the one underlying type. This type never was and never will Zimmermann (1965). Troll (1954) referred to all leaves by the
be realised in a single plant. It is static, immaterial and only organization type ( = foliage-leaf), from which they only differed in
exists as an intellectual construct. Diversity results from their proportions (dotted arrows). He agreed with Goethe (1790) in
dynamics inherent in life. Individual plants are manifesta- understanding 'metamorphosis' to be the variation of an underlying
type. Goethe, however, did not derive the leaves from one another but
tions of the underlying type realised by means of meta- took all to be equivalent (x). In his view, the series of leaves can be read
morphosis, which constantly changes nature. In all cases in both directions (upwards and downwards). Arber (1950) also took
the outer appearance of a plant depends on the stage of all lateral appendages to be equivalent in being partial-shoots. For her,
metamorphosis. The individual forms are real, material and the diverse forms result from parallel development towards whole-
can be studied empirically. shoot characters (solid arrows).
Diversity disguises the underlying unity. Thus, plant
morphology must identify the 'type' behind the diversity. Johann Wolfgang von Goethe
Comparative studies are therefore necessary to ascertain the (1749-1832)
degree of uniformity between organisms. archetypes manifestations
, of the underlying type
metamorphosis
O R G A N I Z A T I O N AND FORM: THE unity ^) • C diversity)
TYPOLOGICAL CONCEPT OF WILHELM dynamic nature
TROLL life of
immaterial real organisms
static dynamic
Troll's study of nature was inspired by Goethe's view of
'unity behind diversity' (Troll, 1928: Nickel, 1996). Troll's
F I G . 5. Goethe's morphology is dominated by the view that immaterial
belief in a universal idea underlying nature corresponded to archetypes exist in nature, which become realised in diverse manifes-
his profound faith in God and determined his empirical tations by the process of metamorphosis. The dynamic force is life
studies (Troll, 1950 p. 561). He devoted his entire life to itself. Goethe's view thus includes metaphysical and dynamic aspects.Clafien-Bockhoff— Plant Morphology 1157
B
D
Sponangien SAM
Phylloide AdvM
re lorn
Mesom \ \ \
LRM
Telom
Mesom
Telom
RAM
F I G . 6. Diverse reference systems used for the interpretation of the plant form. A, Annual plant illustrating Goethe's 'archetype' introduced
by Schleiden (1850) (after Leistikow and Kockel, 1990; the arrows indicate the three steps of metamorphosis). B, The 'organization type' of
the Angiosperm plant applied by Troll, who adopted the scheme from Sachs (1882) (Troll, 1954). C, The 'ancestral telome system' introduced
by Zimmermann (only the terminal elements were called 'telomes': the remaining ones were called 'mesomes'; Zimmermann, 1965). D,
The compilation of vegetative leaf, root and shoot meristems (Kerstetter and Hake, 1997) is taken here as a tentative illustration of Arber's
developmental view. AdvM, Adventitious meristem; A x M , axillary meristem; Co, cotyledon; Gk, terminal bud; tly, hypocotyl; LRM, lateral root
meristem; Pw, primary root; Ra, radicle; RAM, root apical meristem; SAM, shoot apical meristem; w, stem-borne root.1158 Clafien-Bockhoff—Plant Morphology
comparative plant morphology and to the reduction of Although Troll's typology was very successful, it remained
plant diversity to definite types. controversial. Apart from the fundamental criticism
Troll's philosophy had already matured when he concerning idealism (see Zimmermann's view below), the
published his remarkable book, Organisation uncl Gestalt strict distinction of types has been questioned. This is
im Bereich cler Bliite (Floral organization and form; Troll, particularly true for the huge diversity of inflorescences
1928). It deals with the analogy between flowers and which, in many plant families, show transitions from the
inflorescences (Fig. 9) and illustrates the typological funda- monotelic to the polytelic type (Fig. 8B; Weberling, 1989).
ment of Troll's morphology. Troll ignored transitional forms for many years. Only in his
old age, when he had finished writing theoretical papers, is
he said to have accepted transitional forms. Viewed today, it
The organization type f'Bauplan'J is somehow tragic that his principal field of interest
Flowers and inflorescences differ in their organization: (inflorescences) undermined his concept of distinct types
the flower is usually seen as a condensed shoot bearing and, to a certain extent, even his underlying philosophy.
floral leaves, stamens and carpels whereas inflorescences are A more modern view of Troll's 'types' is to see them as
simple or compound shoot systems bearing several to many 'classes' characterized by sharp limits. Nowadays, what
individual flowers. According to Troll, flowers and inflor- Troll called 'types' are seen as 'open types'. They have been
escences differ in their organization ('Bauplan'), because compared with Gaussian curves, which do not exclude
they are manifestations of different organizational types intermediate or abnormal forms (Froebe, 1971, 1986;
(Troll, 1928 p. 25; Nickel, 1996 pp. 44-46). Sattler, 1996). They are no longer considered ideas, but
Troll's 'organization type' corresponds to Goethe's abstract models relating only to the real plants. This modern
idealistic archetype. It likewise represents the unity behind view of morphological types has become the most essential
diversity and is recognized by positional principles (Troll, part of present morphological concepts (see ClaPen-Bock-
1951 p. 376). hoff, 2001; Rutishauser and Isler, 2001) and will be
As regards the Angiosperms, Troll (1948 pp. 123-126; discussed in a future paper.
Fig. 6B) was convinced that they were composed of three
basic organs QGrundorgane'): roots, stems and leaves. Stems
The principle of variable proportions
in contrast to roots are subdivided into nodes and
internodes; leaves are only found at the nodes in a lateral Proceeding from the view that types are clearly separated
position and lateral shoots always arise from the leaf's axil. units which include a variety of individual forms, Troll
The leaf as one of the basic organs is characterized by the concluded that all structures that differ only in their
base, petiole and lamina (Fig. 7B: I). Each part may vary in proportions belong to one type. Referring to Goethe's
its proportions, but all leaves can be compared by referring metamorphosis, he called this rule the 'principle of variable
to the underlying type. proportions' (Troll, 1949 p. 494, 1951 p. 379).
Troll (1964, 1969) was particularly interested in detecting Although Troll (1951 p. 379) included all structures
types behind the diversity of inflorescences. He adopted the differing in quantitative instead of qualitative characters
term 'synflorescence' from his academic teacher Karl von into one type, he did not agree with the transformation theory
Goebel (1931 p. 2), but restricted it to those inflorescences introduced by d'Arcy Thompson (1917). This author started
whose lateral branches duplicate the main axis exactly. As from the purely topographic view. He compared individual
the main axis either ends in a terminal flower (Fig. 8A: II) forms by projecting them into a Cartesian co-ordinate system
or in a non-flowering apex (Fig. 8A: I), only two types and by describing them as mathematical functions. In this
result: the monotelic and the polytelic synflorescence. way he illustrated variable proportions among related species
In the view of Troll, types are clearly distinguished from and analysed the spatial orientation of growth processes.
each other. For that reason transitional forms have to be Troll and d'Arcy Thompson agreed in comparing diverse
forced into one of the given categories. A well-known and forms within the limits of a given framework, the co-ordinate
often discussed example is given by the phylloclade of system and the type, respectively. They also agreed in
Ruscus (Fig. 14D), which looks like a leaf but has the arranging them in a series according to their degree of formal
position of a lateral shoot (Arber, 1925 pp. 138-140, 1950 similarity. However, their concepts were rather different.
p. 97; Cooney-Sovetts and Sattler, 1986). Referring to the While d'Arcy Thompson started from the analytic view and
given basic organs and positional relations, Troll (1937 intended to measure diverse forms in a mathematically exact
p. 347, 1951 p. 378) clearly identified it as a lateral shoot manner, Troll (1928 pp. 19, 25; 1937 p. 11) recognized the
which only looked similar to a leaf (see Arber's diverging underlying type behind the varying forms.
view below). A second example illustrating Troll's cate- In comparing individual forms, Troll predominantly
gorical view is his definition of 'pseudoterminal' flowers took adult structures into consideration. He studied
(Troll, 1964 p. 25). This term refers to a flower developing developmental stages only in order to detect characters of
terminally which, for typological reasons, should be lateral. the adult form (Nickel, 1996 p. 47). Furthermore, he
Examples are found in the monocotyledons in which interpreted the individual forms by deriving them from the
terminal flowers occasionally determinate the actual inde- underlying type. For instance, he concluded that the growth
terminate inflorescence. Troll explained this positional shift form of cacti (Fig. 7A) could be derived from the
as congenital fusion which completely masks the underlying Angiosperm type by reduction of foliage and transforma-
typological relations (see Arber's diverging view below). tion of the shoot into a water reservoir. Similarly, phyllodialClaflen-Bockhoff—Plant Morphology 159
B
F I G . 7. Illustration of Troll's morphology using selected examples. A, The derivation of the cactoid growth form from the organization type of the
Angiosperm plants (Nickel, 1996). B, The organization type of the foliage (I) and the illustration of the 'principle of variable proportions' by the
leaves of Oxalis acetosella (II) and O. bupleurifolia (III-V) (Nickel, 1996). C, Detail of the leaf sequence of Malus haccala showing the bud scale
(left), foliage-leaf (right) and transitional forms. According to Troll, the whole bud scale is homologous with the leaf base only (black), while
Arber argues that the bud scale is equivalent to the whole foliage leaf (dotted frames) (after Troll, 1954). b, Base leaf; Bl, leaf; Co, cotyledon; G,
leaf base; tly, hypocotyl; k, bud; 1, vascular bundle (in A), lamina (in C); p, petiole; s, stipule: Sp, lamina; St, petiole; w, root .
leaves (Fig. 7B) have an enlarged petiole and a reduced be equated with an individual component of another leaf
lamina, while bud scales (Fig. 7C) have promoted the leaf (Fig. 7C).
base at the expense of the remaining leaf parts. Troll's static
view of adult forms suggests that the scale leaf is
homologous to the leaf-base of a fully developed foliage Homology and analogy
leaf. However, Arber (1950 p. 86) rejected this artificial Referring to the example offlowersand inflorescences it is
procedure. She argued, as we do today, from the clear that forms belonging to the same type might be diverse
ontogenetic point of view, that a whole leaf could never while forms belonging to different types might be similar.1160 Cla fieri-Bockhoff—Plant Morphology
F I G . 8. Determinant and open types in inflorescence morphology. A, The two basic, originally sharply limited, types of polytelic (left) and
monotelic (right) synflorescence (Troll, 1964). mF, Main florescence; tf, terminal flower. B, Schematic representation of the processes transforming
monotelic synflorescences (A,) into polytelic ones (B,, B3), with several transitional (A2, A3) and proliferating (A4, B2, B4) forms (Clapen-
Bockhoff, 2000). The main processes are homogenization (//), racemization (Rz), truncation (7), and proliferation (P). These processes show that
the apparently limited types do in fact have intermediates between them, ez, Enrichment zone; psF, pseudo-florescence.
Troll referred to the terms 'homology' and 'analogy' to as identity in structure. He strictly distinguished it from
distinguish between 'identity in type' and 'similarity in outer identity in function ('analogy'). Identical structures were
appearance'. originally recognized by their relative position within the
The term 'homology' originates from pre-Darwinian organism, i.e. by purely morphological means. Since
times. Owen (1848) was the first to define homology clearly, Darwin (1859), however, the terms homology and analogyClafien-Bockhojf—Plant Morphology 1161
F I G . 9. Analogous similarity illustrating the 'Form type' sensu Troll. A, Flower of Ranunuclus (left) and the flower-like inflorescence of Tageles
(right) (Remane, 1956). B, Flower of Teucrium scordhan (left) and the ray floret of Catananche caerulea. C, Petal of Dkmthus seguierii (left) and the
'pseudopetal' (ray floret) of Helianthemwn chamaecislus (right). D, Floral bud of Melandrium rubrum (left) and the prefloral capitulum of Tageles
signata (right) (B-D: Troll, 1928).
have been applied to phylogenetic and ecological aspects. According to his synthetic view, Troll (1928 p. 89) took
Homologous structures in the phylogenetic sense originate Form to be a given entity, which for fundamental reasons is
from a common ancestral stock while analogous structures beyond any analysis. This means that a Form cannot be
are similar because they are equally adapted to functional divided into parts without losing its identity. In the
constraints (see Zimmermann's similar view below). terminology of Arber (see below) it is a 'whole' and not
In view of the influence of ontogenetic and phylogenetic only the additive result of parts. As morphology deals with
processes on form, Remane (1956 pp. 28-93) clearly Form, it is not an analytical discipline, and for that reason
illustrated that the criterion of 'identity in position' was it differs fundamentally from physiology (Troll, 1925 pp.
no longer sufficient to detect homologies (in the phylo- 559, 562). According to Troll, a synthesis between causal-
genetic sense). He added the 'criterion of specific quality' analytical disciplines and typological morphology was
and the 'criterion of transitional forms' referring to unusual impossible, and thus biology is a non-uniform science.
characters and developmental pathways, respectively. Although Troll was clearly influenced by Goethe, he
However, Troll (1925 p. 557, 1950 p. 43) referred to the differs from Goethe in this rigorous point of view. He also
pre-Darwinian pure morphological meaning of the term dissociated himself from his academic teacher Karl von
homology and took identity in position to be the most Goebel (1898 p. 2) who took morphology to be only one of
important criterion. The individual manifestations of one numerous views of biology. Goebel (1898 p. Ill) even
type may show unequal growth of their parts but they always expanded the meaning of morphology to unite it with the
agree in having identical positions within the whole. They arising experimental disciplines. He included developmental
are thus morphologically identical and homologous. As and functional aspects and changed its name to 'organo-
organization is often masked by the design of the adult graphy' (Goebel, 1928 p. 1), thereby emphasizing that
plants, homology has to be detected by comparative studies. morphology was part of modern biology. Troll however
could not accept this view, and resumed the tradition of
idealistic morphology. Thus, he and his followers isolated
German morphology for a long time.
The Form typef'Gestah')
According to Troll (1928), many examples of ana-
Analogous forms differ in organization but agree in their logous similarity e.g. flowers and inflorescences, and
outer appearance (Fig. 9). Troll called the outer appearance marsupials and mammals, clearly illustrate that Form is
'Gestalt', which is here translated as 'Form' (the capital independent of the underlying organization type. Troll
letter indicating its specific meaning). In fact, Form is not (1928 pp. 89-93) supposed that an immanent 'urge to
merely the outer appearance; it is a philosophical concept. Form' CGestaltungstrieb') existed in nature, which found1162 Claflen-Bockhoff—Plant Morphology
its expression in realising only a limited number of given Wilhelm Troll
Forms. (1897-1978)
Troll (1928) found his view supported by his comparative organization
studies on flowers and flower-like inflorescences (Fig. 9A). type
Unfortunately, he called the latter pseudanthia, although I variable proportions,
this term had already been used in another context (see urge to Form
Cla(3en-Bockhoff, 1991). He detected resemblance in unity ^ " ('diversity
minute details e.g. in similar shapes and colours of petals
and ray florets (Fig. 9B-E). He concluded that structures immaterial real organisms
static static (& dynamic)
were much more similar than necessary for an adaptation to
pollinators and that, therefore, Form was likewise indepen-
F I G . 10. Troll started from two given type categories: the organization
dent of functional constraints (Troll, 1928 pp. VII, 89). and the Form type. The organization type is manifested in many real
forms, which differ only in their proportions. The diversity of forms is
limited by the Form type, and is realised by the 'urge to Form'. Troll's
Phytogeny and the 'gradation of types' morphology is characterized by metaphysical, static and limited types.
His idealistic view of typology should not be confused with the present
Typology, in Troll's view, is more a philosophy than a procedure of typology, which corresponds to the generally accepted
procedure. This is clearly documented in his discussion of scientific procedure of abstraction.
typology, biosystematics and phylogeny (Troll, 1951). The
task of systematics is to provide a system offering both a
general view of the diversity of organisms and an image of Troll's typological view of nature
their natural relationships. Referring to the morphological Troll's morphology reflects Goethe's dualistic view of a
systems of his time which group organisms according universal reference system ('organization type') and its
to their similarity in organization, Troll (1951 p. 387) many manifestations ('Form', Gestalt) (Fig. 10). However,
equated 'natural relationship' (naturliche Verwandtschaft) it lacked Goethe's dynamics, as Troll did not focus on the
with 'identity in type' (Ubereinstimmung im Typus). This process of change ('metamorphosis') but on the actual
conclusion followed naturally from his idealistic point of realization of a definite Form. His view was based on two
view, for Troll not only took the types to be strictly
static type categories: organization, which is a metaphysical
separated units, he also found them arranged in an
idea, and Form, which is the perceivable form of an
hierarchical system. Leaflets are part of the lamina, leaves
organism.
are part of the seed plant, and flowers are parts of an
inflorescence. He, thus, generalized that the entire living Unity in organization (homology) refers to given types
world was manifested in a 'gradation of types' reflecting the which are detectable by comparative studies. The represen-
natural order (Troll, 1951 p. 385). The corresponding tatives of a type may be diverse in their outer appearance,
hierarchy of systematic categories supported his view: but they always agree in the relative positions of their parts.
differences in systematic categories correspond to differ- Diversity results from both the principle of variable
ences in underlying types. Troll rejected the idea that proportions (within one type) and the existence of different
systematic categories are the result of common descent, types. Because of the 'urge to Form', representatives of
because his types are invariable constants. To him, only the different types may look similar (analogy).
unity in types could only be interpreted as the fundamental Today, Troll is one of the most controversial botanists in
order in nature. Germany. He is honoured by his followers and insulted by
In contrast to systematics, phylogeny deals with the his opponents. At first, he was extremely successful. He
process of common descent. Although Troll (1951 p. 387) made careful studies of many plants, documented their
accepted the importance of evolutionary thinking, he argued formal characters in clear diagrams and introduced
that phylogenetic systems were impossible because processes reference systems for all parts of the plant (Troll, 1937,
and changes cannot be classified. Instead, morphological 1939, 1943). He provided the first general view on the
studies are always necessary to reconstruct phylogenies. diversity of plant form, and influenced German
Troll concluded that the natural order was only detectable morphology for decades. At least 16 of his disciples have
by typology, and that phylogeny only provided systematics held a professorship in Germany and have passed on his
with the genealogical lines. In contrast to his contemp- typological concept. Later, typology lost its idealistic spirit
oraries, Troll did not accept form-continua contradicting his (Froebe, 1971, 1986; Weberling, 1981 p. 313). It was
type concept. Instead he argued that distinct changes must reduced to a scientific procedure for abstracting general
have happened in the course of evolution to produce discrete rules from the individual. From this point of view, typology
and discontinuous types (Nickel, 1996 p. 57). is a necessary method for the study of plant form.
Troll demanded that each natural classification should be Combined with drawing schemes (side-view, ground-view),
based on types, not on single and arbitrary characters. He it provides a general view of the diversity of plant form, and
thus took typology to be the predominant and fundamental by no means contradicts phylogenetic thinking. Finally,
procedure of systematics (Troll, 1937 p. 48). It is clear that Troll's idealistic spirit became so outdated, and even
Troll's idealistic view, and particularly his view of individual counterproductive, that in the age of increasing phyloge-
forms having derived from given types instead of common netic-thinking heated conflicts with his contemporaries
ancestors, provoked heated disputes (see below). became inevitable. These conflicts were promoted by Troll'sClajien-Bockhojf—Plant Morphology 163
rigid and uncompromising attitude, as well as by the
separation of morphology from biology, by his demand for
the predominance of typology, and particularly by the
mixture of empirical findings and subjective ideas found in
his work.
TELOMES AND E L E M E N T A R Y PROCESSES:
THE P H Y L O G E N E T I C APPROACH OF
WALTER Z I M M E R M A N N
Walter Zimmermann differed from Troll in several funda-
mental aspects. First, he turned against idealism, recom-
mending a strict distinction between object and subject
(Zimmermann, 1937 pp. 6-7). Second, he was a post-
Darwinian phylogenist and tried to reconstruct the
phylogeny of plants according to evolutionary theory.
Thus, he was not looking for the 'archetypal' plant but for
the 'ancestral' plant.
Objects and subjects
In response to the idealistic spirit pervading German
science in his time (see above), Zimmermann (1937) clearly
demanded an objective procedure for all aspects of scientific
research. He did not refuse philosophy, and even confessed
that research might always be partly subjective, but for
practical reasons he tried to separate object and subject as
far as possible (Zimmermann, 1930 p. VI).
Zimmermann (1937 p. 11) explained his view by making Hologenie
the distinction between natural regularities {'Gesetzmd
fiigkeiten der Natur") and natural laws {'Naturgesetze').
While the first are observable and measurable, the latter are
intellectual abstractions and, thus, non-existent in nature.
Nevertheless, as far as natural laws only included relations
that occurred in nature, Zimmermann accepted them as
helpful generalizations and as a necessary part of the
scientific procedure to gain knowledge.
According to Zimmermann (1937 pp. 31-39), abstract
terms are absolutely necessary for communication, but
may imply antithesis. He mentioned, for example, 'life and
death', 'the whole and the parts', 'body and mind', which,
in his opinion, do not exist as antitheses in nature but
rather pervade and imply each other (see Arber's similar Illb
view below). They only appear as antitheses because
practical issues have been mixed up with terms. Guided by
the example 'the whole is more than its parts', Zimmer- F I G . 11. Basic aspects of Zimmermann's phylogenetic view. A, The
mann explained his view. As the whole and the parts are 'hologenetic spiral' illustrating the close relationship between phylo-
geny and ontogeny. B, Reconstruction of Rhynia major from the
not objects, but are actually terms, the question should Devonian, which Zimmermann assumed to be the ancestral vascular
instead be: 'Can we recognize different characters by plant. C, The five elementary processes causing character transforma-
looking at the whole and its parts, respectively?' Zimmer- tion in the course of evolution of vascular plants: I, overtopping; II,
mann (1931 p. 965) easily answered this question because planation; III, fusion in both leaf (a) and stem (b) phylogeny; IV,
cells, for example, contain different biological information reduction; V, incurvation (after Zimmermann, 1965).
to the entire organism. Referring to the question posed at
the beginning of this paper, 'What is a plant form?',
Zimmermann would likewise have modified it to: 'What as the only procedure, which achieves results depending
can we learn about the characters of a plant form?'. The only on the characteristics of the object under observation.
different formulation of the question clearly illustrates the Of course, it is necessary to pose adequate questions and
different views on plant form held by Zimmermann and hypotheses. This is particularly necessary for biology,
by Troll and Arber, respectively (see Fig. 1). Zimmermann which mediates between the inorganic world and the
(1937 pp. 7-8) gave preference to the rationalistic analysis human subject. In this respect, biology is of central1164 Clafien-Bockhoff—Plant Morphology
F I G . 12. The telome theory applied to the phylogeny of leaves (A), inflorescences (B) and stamens (C). A, Reconstruction of the leaf phylogeny in
Magnolia (left) and in conifers (right) showing the assumed ancestral form (at the bottom), its recent form (top) and between them transitional
forms caused by overtopping, planation, fusion and reduction. B, The diversity of inflorescences as the result of character transformation in the
course of phylogeny: starting from the cymose panicle (A), assumed to be the basic form, all the remaining cymose (B-E) and racemose
inflorescences (F— L) are derived by changes in internode length, overtopping, reduction and flowering sequence. C, Hypothetical evolution of
Angiosperm stamens (D-H) from ancestral forms (A-C), which still illustrate their origin in a telome system (after Zimmermann, 1965).
importance because it provides a comprehensive view of types, but analyses plant forms by searching for form-
the world (Zimmermann, 1937 pp. 3-4, 40-42). However, building factors and ancestral forms. As every change in
such a view is only possible if all biological disciplines, plant form is caused by physiological processes, morph-
including morphology, are based on the same adequate ology includes developmental physiology and is even
procedures. related to genetic studies (Zimmermann, 1931 p. 976).
Thus, modern morphology has changed from an irrational
to an empirical-rationalistic discipline.
Morphology and phylogeny
Starting from the undisputed view that the present
Zimmermann's (1930 p. V) main interest was the change diversity of plant form evolved, it is the task of phylogeny
in plant form over the course of evolution. As phylogeny is to identify ancestral forms and to reconstruct genealogies
based on morphology, Zimmermann also dealt with the (Zimmermann, 1935 p. 98). It is also important to
history and significance of morphological studies. From his remember that evolution is a process closely combining
point of view, morphology was a very important discipline ontogeny and phylogeny. Zimmermann (1935 p. 117, 1965
in pre-Darwinian times, but then lost its importance and is p. 31) explained his view by means of the 'hologenetic
presently nothing but a resource for phylogenetical research. spiral' illustrating that each phylogenetic change is based on
Zimmermann (1930 pp. 7-12) summarized three histori- at least one ontogenetic change (Fig. 11 A). Mutations
cal stages in the science of morphology: a descriptive, modify determining factors relating to special characters,
an idealistic and an analytical stage. While idealistic which then appear in the offspring in a changed form
morphology results in a subjective interpretation of (Zimmermann, 1931 pp. 982-983). Such 'character trans-
empirical data, analytical morphology strictly distinguishes formations' represent the fundamental phylogenetic pro-
between the object under observation and the investigator cess. They have to be clearly distinguished from taxon
(see above). Analytical morphology does not look for ideal phylogeny, which results from the knowledge of characterClafien-Bockhoff—Plant Morphology 1165
transformations (Zimmermann, 1931 p. 967). The final aim not known today, Zimmermann's concept remains hypothe-
of phylogeny is to present a taxon tree including all species tical. It agrees with Troll's typology in referring recent forms
and all steps of character transformation (Zimmermann, to one reference, the assumed ancestral form and the type,
1931 p. 982). respectively, and in naming the varying processes and
proportion. However, it differs fundamentally in its inten-
tion to reconstruct phylogenetic character transformations
The telome theory instead of typological i.e. positional relations.
Zimmermann's principal work, Die Phylogenie der
Pflanzen (The phylogeny of plants) was published in 1930.
It includes both an introduction to the so-called 'telome Character transformation
theory' and the broad application of this theory to fossil The present paper deals with morphological concepts.
and recent plants. For this reason Zimmermann's important contributions to
Starting from Rhynia-like fossils (Fig. 1 IB), Zimmer- phylogenetic systematics are not reviewed here; they have
mann introduced the supposed ancestral vascular plant, been reviewed recently by Donoghue and Kadereit (1992)
which was composed of only one element, the telome who clearly illustrated that many aspects of Hennig's
(Fig. 6C). In his view, telomes had a radial symmetry and phylogenetic systematics were based on Zimmermann's
the capacity for branching dichotomously. Adult organisms work. Instead, I focus here on those aspects of character
were three-dimensional telome systems bearing equivalent transformation which affect morphological concepts.
sterile telomes (phylloids) and fertile ones (sporangia). According to Zimmermann (1965 p. 49), the elementary
Zimmermann (1965 pp. 1-7) supposed that the present processes vary independently of each other, and appear in all
plant organs (shoots, leaves and roots) had evolved from possible combinations. Thus, diversity is caused by
telomes in the course of evolution. Thus, the term telome, mutations, which change the phenotypic appearance.
which turns against the 'angiosperm-centred' view, is Though the elementary processes may be reversible in
predominantly applicable to fossil plants (Zimmermann, individual cases, there is a general tendency in the phylogeny
1965 p. 29). of vascular plants to increase the differentiation of the plant
Referring to De Candolle, Hofmeister, Potonie and body, the ability to settle in diverse niches and the amount of
others, Zimmermann (1965 pp. 8-25) listed six elementary useful qualities (Zimmermann, 1930 p. 374, 1965 p. 56). In
processes which underly phylogenetic changes (Fig. 11C): accordance with the theory of adaptation these general
overtopping (unilateral dominance), planation, fusion in tendencies are caused by selection (Zimmermann, 1930 pp.
both leaf and stem phylogeny (congenital switch of relative 400, 421; see Arber's view of 'parallelism' below).
tissue positions), reduction, and incurvation (dorsiventral- As mutations influence the complete ontogeny, it is not
ity). The sixth process is represented by longitudinal adequate to compare adult structures only. Furthermore, as
polarity, which is not however restricted to vascular plants the elementary processes vary independently and change
(Zimmermann, 1965 p. 76). only individual characters in the course of ontogeny,
The telome theory is a phylogenetic theory that illustrates diversity occurs between closely related taxa and similarity
the change of characters from a supposed ancestral form to between unrelated ones. Zimmermann (1965 p. 28) pointed
modern forms (Zimmermann, 1965 p. 30). Since the present out that the same character state might be both primitive
diversity is well known and the direction of the evolutionary and derived in different species and that the appearance of
process is clearly given by time, the main task is to find and primitive character states must not be correlated with the
to fix the ancestral form. primitive or derived state of the entire species. Hereby he
As regards vascular plants, Zimmermann (1930) intro- described the phenomenon of mosaic evolution which was
duced the telome system as the ancestral form and the popularized as 'heterobathmy' by Takhtajan (1959).
elementary processes as changing elements. He applied his Characters dating back to a common ancestral form are
theory to an immense number of fossil and recent plants, and called homologous (Zimmermann, 1931 pp. 944, 989)
to all vegetative and reproductive plant structures (Fig. 12A whereas parallel and convergent structures are of poly-
and C). However, more recent paleobotanical discoveries and phyletic origin (Zimmermann, 1930 p. 379). In contrast to
morphological concepts have restricted the validity of the the original meaning of homology (see above), the term in
telome theory (Stebbins, 1974 p. 144; Hagemann, 1976; its phylogenetic sense relates not only to positional
Leistikow, 1990; Gifford and Foster, 1996 pp. 31-33). identities but also refers to processes and qualities. In
Dealing with inflorescences, Zimmermann (1935) could fact, Albert et al. (1998) recently distinguished 'historical',
not refer to fossils and thus had to infer the ancestral form 'process' and 'positional' homology.
indirectly from generally known facts. He demanded that As regards homologies, Zimmermann (1931 p. 993)
the ancestral form must have been primitive in all characters highlighted the necessity of clearly fixing the ancestral
and found the cymose panicle to agree best with this form. The question of whether a tendril, for example, is
postulate. All other inflorescences might be derived from homologous to a leaf or to a shoot can only be answered
this form by the following elementary processes: inhibition reasonably by referring to a plant whose organs have
of internode elongation; numerical reduction of lateral already differentiated. Otherwise, referring to the telome
shoots and flowers; reduction of leaves; overtop- system, all structures would be homologous.
ping; differentiation; and changed flowering sequence The recognition of polyphyletic character transforma-
(Fig. 12B). However, as ancestral inflorescences are still tions also depends on the reference system. For example, the1166 Clafien-Bockhoff—Plant Morphology
cactoid growth form illustrates convergent evolution Rationalism vs. idealism
because the common ancestor of the taxa concerned was
According to Zimmermann (1931 p. 972), phylogeny and
not succulent (Zimmermann, 1930 p. 379). Only by referring morphology struggle for the same domain. That is, they both
to this ancestral form is it possible to infer that the similar identify and compare characters. In doing this, they use
growth form has evolved independently several times. different reference systems. Zimmermann only accepted the
phylogenetic view and urged morphologists to stop asking
questions that had already been answered phylogenetically.
The necessity of grouping and the different meanings of 'type' However, he did not take into consideration the fact that
morphologists did not intend to reconstruct phylogeny but
In view of the immense diversity of individuals, the wanted to compare plant structures for a practical purpose.
fundamental task of biology is to group organisms, their Zimmermann consequently rejected both Troll's attempt
parts and their life processes. Preconditions for that purpose to apply characters to immaterial types and 'intuition' as a
are clear definitions and exact studies. Zimmermann (1931 relevant method. As regards intuition, Zimmermann (1931
pp. 961-963) rejected the mistaken belief that a form pp. 952-956) distinguished between idealism and recon-
continuum should not be classified. Instead he argued that it struction. While idealistic morphologists refer to arbitrary
would be better to exclude rare intermediate forms than to types, phylogenists have to reconstruct genealogical lines. In
renounce sharp limits. In preferring grouping at the expense so doing they are faced with the problem that phylogenetic
of natural continua he agreed with Troll and differed from lines are not directly observable and that only isolated data
Arber (see below). are available. Phylogenetic conclusions are inevitably
Zimmermann (1931 pp. 942-950) distinguished three abstract combinations (Zimmermann, 1931 p. 983); how-
methods of grouping: artificial, idealistic and phylogenetic. ever, they refer to facts and therefore should not be confused
The former relates to a special purpose and groups, for with intuitive conceptions. Zimmermann's argumentation
example, all assimilating structures. Thus, groupings consist again opposed idealism, and not the typological procedure.
of analogous forms. This method does not refer to According to Zimmermann (1931 p. 950, 1937 p. 25), each
phylogeny, but to some other criterion. The resulting groups grouping that ignores genealogical lines must be artificial.
are practical instead of natural. Having only the reconstruction of phylogeny in mind,
The second way of grouping corresponds to the procedure Zimmermann was of course right. But with regard to the
of idealistic morphology. The types are based on metaphys- numerous aspects of plant form, including functional and
ical thinking and are principally unacceptable for modern developmental constraints, non-phylogenetic approaches
botanists (see above). may elucidate other characters of life. The best examples
The phylogenetic way is based on genealogy. It refers are recent studies dealing with epigenetic regulation and
only to the objects and their natural relations and, thus, in phenotypic diversity (see Schlichting and Pigliucci, 1998).
Zimmermann's view it is the only relevant method for
biology.
Zimmermann strictly distinguished types found by Zimmermann's phylogenetic view of nature
intuition from rational types corresponding to real objects. To understand the present diversity of plant forms,
As regards both artificial and phylogenetic grouping, Zimmermann started from the evolutionary history of
Zimmermann applied the term 'type' to both the type plants (Fig. 13). He was convinced that all vascular plants
specimen of a plant collection and the supposed ancestral had evolved from a RhyniaAike ancestral plant. The
plant. Zimmermann did not reject typology as a procedure, original structural element, the telome, was transformed
but rejected the use of arbitrary types instead of practical into the present plant organs over the course of evolution.
ones. However, as we see it today, the above-mentioned
practical types are no less arbitrary than the morphological
ones. When we remove Troll's types from the context of his Walter Zimmermann
idealism, they prove to be practical types that refer to (1892-1980)
positional relationships and analogous similarities. They do ancestral plant, descendants
not refer to an individual plant, but are abstract models of elementary process
the objects concerned. Zimmermann himself accepted such
character transformation
abstract models in reconstructing phylogenies.
With regards to the different ways of grouping, Zimmer- mutation & selection
mann (1931 pp. 970-975) held the opinion that they hologeny
historic real organisms
complemented rather than excluded each other. Particularly static & dynamic static & dynamic
in those cases in which phylogenetic knowledge is lacking,
he accepted artificial and idealistic groups as preliminary F I G . 13. Zimmermann started from the strictly phylogenetic view. The
views. Although Zimmermann emphasized that in view of present diversity of vascular plants has evolved over time. The starting
evolution the phylogenetic way was the only acceptable one, point was an ancestral plant, which became differentiated in the course
of evolution by the processes of mutation and selection. Zimmermann
he mentioned the ingenious inspirations of Goethe and focused on the change of forms, and declared himself against ideal
Hofmeister that were idealistic at first but were then proven types, but for practical reasons he accepted logical types and
correct by phylogenetic studies. groupings.You can also read
