|
|
Transcribed, by permission, from Tuatara (1984) Vol. 27 (1): 26-48 Principia Botanica: Introduction By Michael heads Abstract Croizat's 1961 book Principia Botanica is reviewed in relation to previous and subsequent work by other botanists. It is shown to be a major contribution to developing effective methods of analyzing and solving problems concerning the geographical distribution, morphology and systematics of plants. Croizat's analysis and synthesis of the factors space (geographical distribution), time (phylogeny) and form (morphogeny, symmetry) in evolution, and how this relates to particular groups of plants is discussed in detail. Keywords: biogeography, botany, carpel, Croizat, homology, leaf, morphogeny, morphology, phylogeny, symmetry.
The main theme permeating Croizat's Principia Botanica (1961) is expressed beautifully in the terza rima from Dante used by Croizat to head the work: In the Principia Botanica Croizat sets himself the task of developing basin concepts which will lead to a deeper knowledge of the quiditate, or true nature, of plant structure. One would receive the impression, judging from most introductory texts, that the problems Croizat deals with, for example "What is a leaf?" and "What is a carpel?", have been answered adequately years ago. Surely a keen student can easily acquire knowledge of a particular plant organ by turning to the glossary often provided in modern texts and find a simple, easily learned definition? Comparative plant morphology defines the various plant organs essentially in terms of given categories. The plant shoot is considered as being fundamentally composed of two types of elements, leaf-like (phyllome) and stem-like (caulome), and the plant root comprises a third element. These elements are imagined to have been modified in many different ways to result in the variety of from which is the subject matter of comparative morphology. The flower, for example, is explained as being an aggregate of modified leaf-like appendages, some of which bear ovules. The morphological research programme which effectively gegan in 1790 with the publication of Goethe's Versuch die Metamorphose der Pflanzen zu erklären (see Arber 1964 for an English translation and commentary) gained momentum rapidly. Over the last two centuries it has led to the accumulation of an immense body of factual knowledge. These facts naturally constitute problems they must be explained. As outlined above, traditional ideology explains a particular organ as a modification of one of the three pre-existing categories. Morphological problems are presented by organs which do not fit easily into the conceptual framework offered by the accepted categories. In these case botanists, who must be technically highly skilled, are often inclined to undertake research on the microscopic structure in particular the vascular anatomy, of an organ in order to solve the problem posted by the question "To which category does this organ belong?" The vascular anatomical criteria used to establish the homology of particular organs with one or other category were largely developed by van Tieghem (1.9 1871). But are the categories satisfactory? Or are there fundamental flaws? Croizat suggests that there are, and over the last twenty years his views have been echoed by thoughtful botanists from many different fields. A fundamental problem with the traditional body of theory is that the categories used in the definition "explaining" the organ are themselves understood poorly, if at all. One example is the definition of "carpel" as a "leaf-like organ bearing ovules". Apart from such thoroughly un-leaf-like carpels as those of Arachis, the peanut, and Hakea with its interfascicular cambia, the definition is of course useless, unless we have a prior, adequate understanding of what we mean by "leaf". The man in the street may feel that a leaf is simply a plant part which is flat and green, but for a botanist familiar with even a small part of the range of plant diversity this definition fails very soon. The fat is that we simply do not have sufficient understanding of what constitutes "leaf" to make the simple homology of "carpel" with "leaf-like organ bearing ovules" mean anything useful at all. Our morphology must be based on a minimum of sound and fundamental concepts, and Croizat argues that this is precisely what is lacking from present day botany, since most authors have been content to simply homologies among themselves completely unanalyzed organs. For many botanists the main problem with the traditional research programme is a practical one it has simply ceased to provide new ideas which can be used with effect to attack old problems. Thus in 1946 Domer wrote: "A striking feature of botany as the science exists at present is the lack of any coherent body of comparative morphological doctrine dealing the angiosperms". Twenty years later Sattler (1966) echoed these sentiments: "Relatively little progress has been made during the last 100 years in the study of the 'comparative' morphology of higher plants." This is surely cause for a reassessment of the bases of our knolwede and, even more importantly, our methodology. Of course, the traditional research programme is by now such an integral part of botanical training and research that it would appear to be extremely difficult to change it. Meeuse (1966: 29) has stated: "Both Takhtajan…and Eames…* [*both botanists of the traditional school] whilst adducing corroborative evidence in their argumentation, minimize or misrepresent contradictory findings and often dismiss dissident opinions and alternative theories with a summary self-assurance that verges on contempt. They write with so much self-assumed authority that that inexperienced student may be completely overawed and is likely to accept their apodictic statements as the 'last word' in a particular branch of botany. Adoption of scientific principles by accepting the teachings of an 'authority' was a medieval ('scholastic') tradition Galenus dixit' it is certainly not promotive of educating a budding scientific mind towards independent thinking." And in a sympathetic review of Meeuse' book Cutter 1966 has stated, rather more diplomatically but to the same genera end, that: "Plant morphologists, in general, tend to be a somewhat conservative group. One has only to read the reception accorded to the hypothesis put forward in 1908 by W. H. Lang, on alteration of generations to ceded that, historically, novel viewpoints in their field have tended to encounter opposition". Despite this conservatism and invocation of authority, both hardly conducive to scientific progress, there is every indication that the botanical literature of the past decade reflects a growth awareness of problems raised in the Principia Botanica, and also of the value of many of the suggestions made in Croizat's work towards improving the situation.
It will be disconcerting to many botanists that Croizat's analysis of plant form begins with a thorough analysis of the dispersal of the carnivorous plants and their allies. Step by step Croizat methodically establishes that Droseraceae, Nepenthaceae, Sarraceniaceae, Lentibulariaceae and their relatives have, in fact, evolved stressing the same ancient, cardinal biogeographic nodes as angiosperms development itself. This conclusion is totally novel and thus, of course, unexpected, but it is based on a well documented argument which, while being lengthy and complex, is logically reasoned. Rather than agree or disagree with the conclusion on the spot the reader would be best advised to read Chapter 2 of Principia Botanica first, and then decide where he or she stands. The dispersal of carnivorous plants (for example the striking vicariism displayed by Nepenthaceae with respect to the main massing of carnivorous forms in Australia, and also of the closely allied Dioncophyllaceae in West Africa) indicates that these taxa are not derived independently from other extant angiosperm families, as is commonly thought, but are instead the result of differentiation of a widespread ancestor. The actual tracks features in their dispersal (for example Usambara (Tanzania)-Madagascar-Drakensburgs (South Africa); Duida-Roraima (Venezuela)-Futadjallong (Guinea)-Cameroon) point to an ancestor as old as angiospermy itself. As Croizat states, these ideas can be related closely to those of Huxley (1888) on the ancestry and evolution of Gentiana. Nevertheless, Croizat's analysis is considerably more extended, and provided early evidence for the geologically hybrid nature of North America (Princ: Fig. 8, p 79 and see p. 11 this issue) (geological heresy in 1961, but see Nur and BenAvraham, 1982), as well as providing strikingly novel interpretations of carnivorous plant evolution in space and time. Maguire and Ashton (1977) have commented that Croizat's work is "studied with prescient insight" but in a review such as the present it is perhaps more important to emphasize Croizat's method rather than apparent "prescience" his conclusions are due, after all, to effective analysis rather than lucky guesswork! It is traditionally assumed that the carnivorous plants form an assemblage that is "artificial" in evolutionary terms, resulting from convergent offshoots of various extant taxa such as Saxifragaceae, Scrophulariaceae etc. This assumption is a consequence of essentialistic thinking in morphology whereby leaf, stem and root comprise the original irreducible categories. Notorious among traditionally minded botanists are carnivorous plants such as Utricularia (Lentibulariaceae) in which it is often impossible to distinguish organs corresponding to any leaf, stem or root. The pre-evolutionary morphologists, with their a priori notions of the fundamental nature of the categories leaf, stem and root, felt that the ideal angiosperm would naturally display these organs in their purest, most unsullied for. Thus pre-evolutionary doctrine regards these plants and their organs as degraded-to the extent that in them the pure morphological categories are no longer recognizable. That these plants must cause discomfort is evidenced by the infrequency with which they are discussed and illustrated. The transition from pre-evolutionary botanical thought to the acceptance of an evolutionary interpretation must be recognized as a great conceptual advance. But the necessary revisions of all aspects of botany were not carried out. In fact, as Sattler (1974a) has established, botanical research and teaching is plagued still by the pre-evolutionary essentialistic morphology. Nowhere is this more clearly evident than in the maintenance, albeit disguised, of the dogma of leaf, stem and root as fundamental and primal. This dogma is itself perhaps nowhere more firmly established than in the conventional interpretation of the "derivative" nature of the carnivorous plants. This pseudoevolutionary interpretation proceeds as follows: Given that in angiosperms leaf, stem and root are 'primitive' (the old concept in its new guise), and given that many carnivorous plants and their allies show not distinction between these categories, then it follows that the carnivorous plants must be derived. But as, Croizat contends, any evolutionist must agree that the first premise of the argument is unwarranted. In fact, Croizat's analysis of the carnivorous plants indicated that many of them, and their allies, e.g. Podostemaceae, represent an early level of angiosperm evolution at which the modern leaf, stem, root distinction has not yet been attained. Over recent years evidence has continued to accumulate which points to the consanguinity of the carnivorous plant assemblage. For instance, Marburger (1979) examined the microscopic structure of the stalked and sessile glands of Triphyophyllum petatum (Dioncophyllaceae). Unlike gross morphology, this is an aspect of anatomy least likely to show convergence if the groups are truly unrelated. Nevertheless Marburger found that the structure of the glands were "remarkably similar" to those of Drosophyllum lusitanicum of the Droseraceae. Likewise, Adams & Smith (1977) have illustrated the many morphological similarities, both gross and microscopic, among the five genera of pitcher plants. Lentibulariaceae is closely linked with several other carnivorous groups through its very diverse members. The whorled Utricularia tabulata connects the family with the Hippuridaceae. Through the "U. avesicaria" group Lentibulariacae is allied with the Podostemonaceae. The pitchers and traps of the U. "dicotoma-mananthos" aggregate, as well as Genlisea and Cephalotaceae establish morphogenetic links with the Nepenthaceae. At this level of evolution the transition from floral zygomorphy to actinomorphy is easily achieved for instance through Cladopus, Dicraeia, Inversodicraea, Castelnavia, to Jenmaniella, to Dalzellia, Apinagia, Loncostephus and Tulasneantha all in Podostemonaceae. Thus, the zygomorphic flower of Lentibulariaceae is not at all necessarily remote from the actinomorphic flowers of Droseracae, Nepenthaceae and Dioncophyllaceae the carnivorous plants are connected rather directly with 20-30% of angiosperm families. For example, Airy Shaw (1951) and Schmid (1964) provide evidence for the affinities of the Dioncophyllaceae with the huge assemblages around Flacourtiaceae, Guttiferae and Droseraceae. The Podonstmonaceae are vegetatively closely bound with the Hydrostachyaceae. Florally the former family can easily be interpreted as the result of the reduction of the unisexual inflorescence of the latter family to a lone ovary with the addition of a few stamens. Together these two families furnish the minimum common denominator for a vast range of angiosperms. For example, the close affinity of Hippuridaceae and Halorrhagidaceae is widely acknowledged the fusion of four Hippuris carpels is clearly competent to produce a normal halorrhagidaceous flower. Thus Huppuridaceae form a morphogenetic link between Podostemonaceae on the one hand, and Halorrhagidaceae, Gunneraceae, Lythraceae and Onagraceae on the other. Lentibulariacaeae and Droseraceae are bounde by Byblidaceae (and Aldrovanda) which also serves to bring together Cheiranthera (Pittosporaceae), and actinomorphic Pittosporaceae and the more or less zygomorphic Ochnaceae. The conclusion of Croizat's analysis thus point to the carnivorous plants and their allies as representing a level of evolution underlying all of modern angiospermy, and not at all as a derivative group. It may be pointed out here that unless the botanist is prepared to take Croizat's conclusions on faith, which is hardly what the author intended, he must first gain a knowledge of the diversity of plant form rather wider than that taught in degree courses at most universities. An excellent way of achieving this basic familiarity is through living and studying in the vicinity of tropical rainforests, although this must be supplemented with time in the herbarium, garden and library. Illustrated floras of large tropical regions epitomized by the Flora Melesiana, are invaluable. One or two volume synopses of angiosperm families are of limited use only, works by Baillon (1871-1888) and Engler & Prantl (1924-) give a much more satisfactory impression. Croizat's sound advice to the student is clear - look hard and long at the plant and the pictures, and ignore written description, to begin with, as much as possible. ANALYSIS OF PHYLLOTAXIS AND SYMMETRY The word symmetry is often used in modern times to mean radial and even, as Weyl (1952) has noted, bilateral symmetry. The science of crystallography places no such restrictions on the idea of symmetry, and if development of general concepts of biological symmetry is desired we must likewise avoid any tendency to thing that radial/bilateral symmetry is the only important mode. The study of biological symmetry is obviously of great importance, but apart from work on phyllotaxis (leaf arrangement) it has been generally overlooked. For this reason biology has nothing to compare with te sophisticated work attained in the description and interpretation of crystal structure (Phillips 1971). This has had a crippling effect on the development of biology and on the flow of ideas between the sister sciences of biology and geology, in much the same way as has the lack of efficient concepts in biogeography. As one example of a general, fundamental problem of biological symmetry we can examine the question “why to components of organic structure so often possess symmetry based on five”? It is often observed that among flowers symmetry of five is the most frequent. It is less well known that sometime around 1510-1516 A.D. Leonardo da Vinci determined that in many plants the sixth leaf stands above the first (Richter 1939), this being perhaps the first reference to what later became well known as 2/5 phyllotaxy (the system consists of repetitions of five leaves in two turns of the axis). This is the most common of all patterns of leaf arrangement. In the animal kingdom symmetry based on five is manifest rather less obviously, be even so recurs with such frequency as to constitute a phenomenon of general interest. The radiolarians furnish many forms with pentagonal symmetry will come as no surprise to those familiar with these beautiful animals. Examples include the Pentaspheridae, the Pentinastrum group of general in the Euchitoniidae, and Cicorrhegma (Circoporidae) (Campbell 1954). The foraminiferan Pentellina pseudosaxorum exhibits a pattern of growth idetnifical to phyllotaxy in mode 2/5, sectors of growth being separated by a difference of five members (Van Iterson 1907; Croizat 1964a 440). The Priapulida, a group of burrowing marine worms, possesses dental armature arranged on its proboscis in pentagonal whorls (Nichols 1967). Echinodems, of course, sho a striking adherence to pentamerism. Since the time of the earliest known amphbians 360 million years ago, five has been the dominant number of digits in tetrapods, reductions from five (e.g. horses and birds) have been frequent, but increases occurring hardly at all, and then constituting abnormality. The “biological rule of five” is discussed only seldom (Nichols, 1967, has discussed it with reference to animals), but Croizat, in Chapters 7 and 8 of the Principia Botanica has subjected it to a thorough analysis. D’Arcy Thompson (1917) in a very influential work, unfortunately failed to realize that the key difference between the main modes of phyllotaxis is simply one of superposition. For instance in his fig. 327 “leaf” 1 is clearly superposed by “leaves’ 14, 9, 6, 4, in Fig. 327a, b, c, d respectively, leading to modes 5/13, 3/8. 2/5 and 1/3. In describing the obviously distinct appearances of the phyllotactic modes Thompson failed to mention this, noting instead that “the mathematical side of this very curious phenomenon I have not attempted to investigate”. This simple oversight led to over half a century of deep confusion, with many authors bent o analyzing the mathematics of this evidently very complex subject! As with the general problem of biological symmetry, the concept of superposition must be primarily biological rather than geometrical. Biological superposition cannot require, as does geometrical superposition, the presence of an exact perpendicular. The analysis of biological symmetry which utilizes essentially geometrical premises is similar in many respects to the analysis of plant morphology which begins with the given categories of leaf, stem and root or biogeography which begins with casual migrations. Utilizing the concept of superposition, the symbolism “2/5 phyllotaxy” refers to an important biological reality. Phyllotactic modes of ½ (two leaves per turn) and 1/3 (three leaves per turn) represent the morphological consequences of a meristem producing the minimum number of primordial (two and three) in the lowest modes of symmetry biradial and triradial. The minimum number of systems which can interact is, of course, two. Beginning with the two systems which themselves represent minimal symmetry, Croizat’s analysis shows that an immediate result of interaction between ½ and 1/3 symmetry is, in fact, 2/5 symmetry, or symmetry in fives. As with 2/5, the other common modes of phyllotaxis, 3/8, 5/13 etc., also produce between two and three leaves per turn i.e. ½ and 1/3 mark the basic symmetries. Croizat concludes that whenever a system of many primordial evolves by reduction of parts (e.g. by fusion) or by increase in number of parts, the system will tend to the minimal symmetries of ½ and 1/3, and their first “sum” 2/5. This tendency is responsible for the establishment, at the level of coelocanthid fishes, of the morphogenetic premise which led for example, to the five fingers of Homo and for the reduction of the ancestral strobile around five sectors of growth which led to modern pentamerous flowers. The genetic spiral, described by the developmental sequence of leaf primordial, is often assumed to have special significance for phyllotaxis. But by a constant, gradual displacement of primordial, phyllotaxis may pass easily from, say, mode ½ to mode 2/5. This is possible simply because both modes have two spirals of growing points (cyclosectors), just as modes 1/3 and 3/8 both have three. Thus the genetic spiral, while being descriptively useful, is interpretatively useless, since all phyllotaxies are composed of more than one cyclosector. The evolutionary history of echinoderms would furnish excellent material for a study of the inter-relationships of minimal symmetries. Biradial, pentaradial, and possibly archaic triradial forms exist. Current debate is concerned with whether or not this trimerous stage occurred in ecinoderm evolution (Philip 1979, Stephenson 1979), but unfortunately none of the students involved have related the problem to general concepts of biological symmetry. Modern studies of symmetry and ontongeny have rejected the traditional reliance on adapationist ‘explanations” of aspects of organic form. For instance Goodwin (1982a, b) interprets developing organisms as: “entities with an extensive range of morphological potential, describable in terms of probabilistic fields which collapse…into specific morphologies”. (1982b: 52). These concepts parallel those of Croizat on morphogeny vs. morphology very closely indeed, which is interesting as they have emerged from what are usually regarded as distinct areas fo study ontogeny and comparative morphology. For Goodwin the probabilistic field properties are a function of “general organizational principles”. (1982b: 53). As a consequence of this fundamental change of emphasis Goodwin has subjected the neo-Darwinian approach to a critical and severe analysis, and concluded: “Once it is recognized that there are principle so of organization and laws of form in biology, these time-independent properties of the living realm become once again central to the subject…. The realization that genes do not generate biological form leads to a rather different view of the evolutionary process in terms of the potential forms of the organisms and their appearance on the earth.” (1982a: 111-112). Thus Goodwin, Like Croizat, would place the emphasis on Darwin’s “laws of growth”, in contrast with the neo-Darwinists tradition of virtually ignoring them. Working with rather different subject matter from Goodwin, the Russian palaeobotanist Meyen (1973, 1978) has argued at length for a greater emphasis on the study of “general structural principles” (again corresponding to Darwin’s “laws of growth”) independent of phylogenetic considerations. Baas (1982), discussing the evolution of wood anatomy, criticizes “ridged adaptationist interpretations”, advocating the important role of “functionless trends imposed by correlative constraints…”. INTERPRETATION OF LEAF AND STIPULE The question of "What is a leaf?" is certainly one of the great problems of botany. As mentioned above, the current state of despair is well exemplified by the comment: "Although no satisfactory definition of a leaf is thus possible I shall assume that we all know what we shall be talking about (Gregory 1956). It is clear that the definitional approach, using morphological, especially anatomical, criteria has failed to supply us with efficient concepts involving the true nature of the leaf. If we agree that the assumption "we all know what we shall be talking about" is hopeful but unfounded, we must also agree that it is necessary to do something. It is Croizat's connection that descriptive anatomical criteria cannot supply an answer to the question "what is a leaf", and are, in fact, an inappropriate basis for evolutionary studies. In support is the comment by Schmid (1972: 442) that it is Croizat who provides: "the most pertinent and clear statement I have encountered regarding the problem of the validity of vascular conservatism". Croizat asserts that the most useful analysis of "leaf" would be in terms not of unanalyzed morphological homologies, but rather of morphogenetic processes. With respect to Croizat's attitude towards interpretation of morphogeny vs. technical description of morphology, it is interesting to note Einstein's view that" "It is really strange that human beings are normally deaf to the strongest arguments while they are always inclined to overestimate measuring accuracies." (from a letter, quoted in Feyerbrand 1978:58). Croizat's analysis of leaf form and morphogeny (Princ. Caps. 9. 10) includes consideration of many structures which were traditionally regarded (if at all) as unusual, insignificant, and, ultimately, accidental. Arber (1934: 312) has summed up this unproductive attitude well: "Another dictum of formal morphology is that the power of producing lateral shoots is confined to axes. When a leaf does, in fact, bear a shoot-bud, this shoot is described as 'adventitious', which means, literally, 'accidental'. This is a typical example of the tyranny exercised by words over thought; just because they have themselves labeled these buds 'accidental', botanists feel justified in dismissing them as of no morphological significance." Just as 'inexplicable", thus "accidental", patterns of dispersal (such as trans-tropical-Pacific) are analyzed in his Panbiogeography, Croizat does not fail to consider the nature of "inexplicable" and "accidental" morphological facts in the Principia Botanica. The morphology of plants with structure problematic in the light of traditional interpretations has often led students, for example Jong and Burrt (1975) working on Gesneriaceae, to reject the "traditional morphological categories" as adequate guides for efficient analysis. In Croizat's work these "difficult" plants are not ignored, on the contrary, as with incongruent area/taxa cladograms in biogeography, they are shown to be in themselves useful guidelines in analysis. The problem of the modern leaf was eliminated in the past, when "leaf" was proposed to be itself an irreducible category, an elements, and essentially simple structure. The sociological and philosophical reasons for this proposition are complex suffice to say that Croizat begins with no such assumption. Indeed, but means of a thorough analysis of leaf form, Croizat demonstrates that, in fact, the modern angiosperm leaf is an essentially compound body (c.f. his analysis of Wallacean areas of endemism as compound entities). We are all aware of the complexity of leaf organogenesis, attained by a variety of meristems (Jeune 1981) and Croizat's conclusion may initially seem relatively innocuous. Nevertheless, it has important consequences. Croizat approaches the morphogeny of the modern foliage leaf, stipules, cataphylls, buttress, buds and certain types of thorn (Cactaceae, Euphorbia, Fouquieria) as a general problem As with the leaf/shoot question, rather than simply providing homologies between unanalyzed organs it would seem more productive to inquire about the limits between the organs, thus identifying processes of divergence. Croizat's analysis concludes in fundamental agreement with Tyler (1897) that the "lower foliar organs" (stipule, cataphyll etc.) are neither reduced leaves fro additions subsequent to the development of the modern leaf, but are the "primitive foliar organs". The modern leaf represents a development of, and upon, this primitive leaf. The lateral portions of the primitive leaf, when separated, form the stipules, petiole wings etc, of the modern leaf. The sheathing petiole is a product of the development of lateral and central basal parts of the primitive leaf it is essentially distinct from the true petiole of the modern leaf. The leaf buttress represents the immediate continuation of the primitive leaf into the cortex. These ideas are supported by studies of leaf organogenesis. For example, Cross (1938) has shown that in Viburnum cataphylls are not leaf homologues - cataphyll and leaf ontogeny diverges dramatically at the 80ì stage and leaf growth progresses by means of a distinctive ventral meristem which cataphylls lack. Bruck and Kaplan (1980) have shown that scale-leaves are not homologous with foliage leaves in Muehlenbeckia. In Morus, Cross (1937) found that the stipule development is unlike that of foliage leaves, but like that of cataphylls. Again the difference is fundamental, the foliage leaves being the result of the activity of a ventral meristem totally absent in the cataphylls and stipules. Thus the primitive scaliform leaf, represented by stipules and cataphylls, and the modern foliage leaf represent essentially distinct variants, at a very low level, of a single foliar type the scale of Croizat. In the case of the foliage leaf, this scale or undeveloped phyllomes has inherited a specialized "dab of meristem" which, once activated, leads to leaf organogenesis. Macdonald (1981) stated recently that "The phylogeny of the stipule remains unresolved", indicating that little, if any, conceptual progress has been made since Sinnott and Bailey (1914) made similar comments sixty seven years earlier. Macdonald also noted that: "to conclude that the stipules of Comptonia (Myricaceae) are lobes or outgrowths of the leaf base, while true in an empirical sense, contributes little to our understanding of their phylogeny". However, recent work by Jeune (1981) concludes that, in the light of current knowledge of leaf ontogeny, Croizat's analysis is of special significance in adequately resolving the morphogenetic and phylogenetic nature of the stipule. Macdonald concluded his perceptive paper by suggesting homology between the stipule and prophyll (he unfortunately overlooked Croizat's analysis of "prophyll", and his identification of it with "stipular sector" in Princ. 718). Croizat's synthesis of the morphology of the modern foliage begins with verticals or whorls of primitive "leaves" (to be regarded more as phyllomes or primordial). As Howard's (1974: 160) has noted: "The evolutionary progress from alternative and spirally arranged leaves to opposite or whorled leaves has become established as a dictum inmost botanical publications…without any real evidence." Howard (1974: 163) concludes that "the primitive leaf was probably borne in whorls or even vertically grouped clusters…" These verticils, each of n "leaves" became suppressed, recombined and dirempted, or pulled apart, along the shoot into local sectors of 1 foliage leave + 2 stipules (undeveloped leaves). This process of reduction naturally takes place within the limits imposed by laws of symmetry, for example those outline above. What is the explanation for the development of the single leaf as the keystone of reduction and diremption of the ancestral verticil? The stipule/leaf distinction is not adequately defined by the presence of an axillary bud (e.g. Chaenomeles japonica stipules may have axillary buds), but histologically the distinction cab be made on the basis of the ventral foliar meristem. The crucial question thus becomes "What is the phylogenetic and morphogenetic nature of this important meristem? What is its origin?" The only real answer given it this question is that of Croizat. The meristem is the product of ancient fusion of axes and primitive "leaves" leading to the modern leaf. The meristem itself represents the incorporation into the primitive "leaf" of the primordium of the ancient axis. Fundamental to Croizat's analysis is the nature of the axillary buds observed in modern plants. Although some plants may have only one bud per axil, in principle there may be a series of axillary primordial lined up between the petiole and the axis. Today these are responsible for shoot making of various kinds (floral and vegetative) including the fusion of epiphyllous inflorescences and leaves. In the history of the leaf, one element of these primordia has been competent in fusing an axis with the ancient "leaf" leading to the meristem which characterizes the "modern" leaf. The relationship between the primitive scaliform "leaf" and the axis which it subtends and later fuses with is essentially hypocladial, in the sense of Kursner (1954). Hypocladial relationships between rameal and foliar organs are well known in many extent plants, for example, see Fig. 2. The axis which has become reduced and incorporated into the primitive scaliform leaf, leading to the development of the modern, hypocladial leaf could naturally be expected to have left some indications as to its nature in other words the hypocladial transfusion may well not have been completed in every instance. Dickinson (1978) in a timely and thorough review of the phenomenon of epiphylly has urged the study of the laws of growth which have determined the development of epiphyllous inflorescences. He has also noted the possibility of epiphylly having been of "fundamental importance…to the origins of the angiosperm leaf itself" and suggests that this concept provides significant avenues for future research. The idea of the inherent sexuality of angiosperm leaves can be traced back to at least C. de Candolle (1890) but its phylogenetic and morphogenetic meaning is first explained by Croizat, in terms of the hypocladial nature and history of the modern angiosperm leaf. Epiphylly is, of course, a worry for traditional morphology which regards the leaf as an irreducible category - the consequent problem lies in explaining how and why an inflorescence managed to "get up" into a leaf. However, with Croizat's analysis of the leaf as essentially complex, the "problem" of epiphylly vanishes, it simply means that the axis involved with the primitive scaliform foliage was, intact, or potentially, floriferous. Thus the hypoclaidal relationship between (potentially) floriferous axes and the primitive scaliform foliate is an important law of growth for the modern angiosperm leaf. In a study of epiphylly in Helwingia japonica Dickinson and Sattler (1975) state: "The main conclusion is that such conditions (epiphylly) suggests that "laws of growth"…probably are as important, or more so, than lows of natural selection in determining plant form." The same authors (1974: 8) in a study of the epiphyllous inflorescence of the saxifagaceaous Phyllonoma intergerrima state that: "our observations cannot be incorporated into the rigidly formulated classical theory of the shoot without distortion of the observations themselves, or of our understanding of them." They also note that (1974: 9) "The value of atypical situations like epiphylly is that they point out aspects of the real morphogenetic potential of plants in nature, corresponding to these "Laws of Growth", that we often overlook." This potential may lie at no great depth, for example Stebbins (1965) describes the single "gene" controlling the production of epiphyllous inflorescences in pants of Hordeum trifurcatum (see Princ. 1533). A question of crucial significance for an understanding of the leaf is posted by the nature of glands. Most recent work concentrates on their present day ecological significance and tens to ignore their morphogeny. But as Schnell (1970: 433) has said, regarding etra-floral glands: "Leur signification…parait a rechercher dans la morphologie et dans la phylogénie de la feuille plutôt que dans une utilité pour lat plante…"¹ [¹ "An understanding of their significance would seem to require research into the morphology and phylogeny of the leaf, rather than into any usefulness for the plant."] Since Schwend's important 1907 paper, botanists have interpreted the morphology of plant glands as a result of the reduction of the hemming-in of pre-existent structures (e.g. Schnell 1969: 153-154 for discussion). Glands, often secretory, are well known from the vegetative parts of many plants, as well as from the flowers. Particularly common are glands found at, or near, the petiole-lamina junction. Persistent meristems fo various kinds have also been reported from this locality (e.g. Jong 1973 on Streptocarpus). Glandular teeth are well known on stipules (e.g. the rubiaceous Coprosma) and leaves (e.g. Chloranthaceae). The interpretation supported by Schnell (1969) with respect to domatia found in the axils of foliar venation (again Coprosma provides striking examples). He suggests that they indicate (une croissance avortée". With respect to early theories on the origin of ant domatia by means of natural selection, Philipson (1964) cites Bailey (e.g. 1922, 1923) favourably, to the effect that: "insects are not concerned with the origin of development of these structures". Thus there are many different structures present in the morphology of the modern foliage which represent remnants of hemmed-in growth, and which must all be accounted for in any synthesis of leaf morphogeny. Howard (1974), in an important contribution, discusses aspects of nodal and petiolar form and concludes that these provide further support for Croizat's concept of the leaf and its essentially compound nature. INTERPRETATION OF THE ROOT The question of the seed plant's root has been answered even less satisfactorily by descriptive morphology than has been the question of the leaf. Descriptive criteria fail dramatically when faced, for example, by Lentibulariaceae, and in fact, there are virtually not concepts efficient in producing even minimum of understanding. In Chapter 11 of the Principia Botanica Croizat begins his discussion of the root with an analysis of the structures found at the junction of hypocotyls in the seedlings of various taxa. These structures (known in the literature as "wurzelhals", "foot", "peg", "collet", etc.) are much more widely distributed throughout angiosperms than was formerly realized, occurring in such taxa as Hippuris, Curcurbitacae, Triglochin, etc. In Eucalyptus erythrocoris the normally horizontal collet forms a sheathing structure morphogenetically identical to the graminaceous coleorhiza. In the embryo of seed plants, root and plumule, representing contrasting polarized centers of development are joined by a transitional zone, the hypocotyls. Beyond the root intials lies the meristem (e.g. the rib meristem of Pseudostuga Allen 1946) from which arises the root cap. These three embryonal meristems are all bound within a jacket of ground tissue, Croizat's synthetic concept which comprises a sheath of considerable morphogenetic powers. The jacket is responsible for the development of various structures, for example tubers, napiform taproots, the collet, the haustorium of parasitic plants, the holdfasts and coralloid roots of the Podostemonaceae, the grass coleorhiza, and of course the rather inconspicuous cortical layer surround the hypocotyls in other "higher" plants. The concept of "jacket" is an efficient tool for rationalizing many aspects of root morphology from Pseudotsuga to Triticum, to Utricularia. But a fundamental problem is offered by such plants as Podostemon ceratophyllum which have no distinct root, but do possess a root cap. In fact Croizat recognizes the problem of root cap as of fundamental significance. He suggests that it is advisable to maintain a general concept of rhizophore for what is currently terms "root", "rhizome", "runner", "hypocotyls", "pneumatophore" etc. in opposition to one of root cap. Thus the rhizophore is seen essentially as the bearer of the root cap, and structures such as the underground axis/rhizophore plus roots of the form genus Stigmaria, and the corm (rhizophore) plus roots of Isoetes, long considered problematical, are simply interpreted as relatively undeveloped morphologies in the morphogeny leading to angiosperm roots. (c.f. Stewart 1947; Sporne 1975: 69-70). Croizat's analysis has not been developed by modern workers on theroot, but it is clear that this is the least understood plant organ of all, and basic morphogenetic concepts are urgently needed. Too many recent studies of root form still provide "explanations" of that form in term son of function and adaptation! Invariably aspects of root morphology apparently restricted to a small number of taxa are regarded as "advanced", "derived" etc., homologized with something or other, and forgotten about. A good example is provided by the "macrodpodous" condition in certain monocotyledonous embryos (see Dahlegren and Clifford 1982: Fig. 8). OTHER BOTANICAL CONTRIBUTIONS This review has so far mentioned some of Croizat's work on the fundamental of botanical philosophy, but it would be unwise to ignore altogether his contributions to other aspects of botany. Croizat felt it was crucial to be able to place work and ideas in their historical context. In the Principia Botanica he seldom considered this as an end in itself, but the importance he placed on it and the interest it held for him can be seen in various other publications dealing largely with historical questions (1945) and biography (1949a, b). Most of Croizat's early work deals largely with the taxonomy and nomenclature of Cactaceae and Euphorbiaceae, and is of only minor interest to the botanist who does not have a special interest in and knowledge of these families. In much of his work it is interesting to observe the enthusiasm with which Croizat deal with the horticultural aspects of his favored plants (e.g. 1941a). Practical rather than theoretical question provided the impetus for all of Croizat's work which was not done in a vacuum but is of interest to all laymen and scholars who ask the question "Why is this plant the way it is, where it is?" It goes almost without saying that Croizat was a formidable enemy of ivory-tower scholasticism. Because of the practical necessity of supplying a name for a plant, Croizat was naturally concerned with the correct application of the rules of botanical nomenclatures, and also took an active interest in the development and improvement of the rules themselves (e.g. 1941b, 1953). Croizat has published several large and important botanical works since the Principia. In Croizat (1964a, b) his ideas on angiosperm evolution in space, time, and form are summarized and, to an extent, developed and refined. Croizat (1965, 1967, 1972a, 1973a) provide both an overview and a detailed analysis of Euphorbiaceae, Euphorbieae and Euphorbia and represent conclusions reached after several decades of cultivation and study of these plants. Croizat's work on these taxa is undoubtedly among the most important yet produced. Croizat (1970) is a critical interpretation of Corner's fascinating but almost totally neglected Durian theory of the origin of the modern tree. Croizat (1971, 1972b, 1973b) are very important contributions to the study of the leaf and of phyllotaxy, in which a considerable amount of mew material is introduced. Although the most striking aspect of the Principia Botanica is undoubtedly the fundamental nature and originality of the principles developed, the reviewer would be failing in his task if he did not attempt to place Croizat's botany in some sort of historical perspective. As with his biogeography, many of his ideas were hinted at, though scarcely developed by earlier students. Cusset (1982), in which must be regarded as a landmark of botanical historiography, has produced a reviewer of the conceptual bases of plant morphology. In it the affinity of Croizat's concepts, particularly concerning foliation, with those of botanists such as Warming and Trécul is underscored. In the fields of floral morphogeny and high systematics Croizat's conclusions often reflect more his sympathy with those of Baillon, a botanist neglected by modern students. Many of the principles established in Croizat's work are becoming accepted, or tat least discussed, by the botanical community, but too often the work is cited only by leading researchers examining fundamentals of botanical knowledge. Obviously even the reading of a large work such as the principia is a major undertaking much time is required to digest the arguments and check up on examples in field, herbarium and literature. But probably the main reason for the surprisingly small number of papers discussing Croizat's ideas is the fundamentally heterodox nature and the complexity of the analyses. As his ideas become more acceptable to the botanical community more credit will undoubtedly be given Croizat for the great significance of his contributions to the "Beginnings of Botany". References Adams, R. M. II and G. W. Smith, 1977: An S.E.M. survey of the five pitcher plant genera. |
