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The Natural Philosophy of Plant Form by Agnes Arber


Goethe's whole conception of plant structure, which we have considered in the preceding chapter, is permeated by the idea of the type. He offered, as an intellectual basis for vegetable morphology, his notion of the prototype plant (Urpflanze)-the common idea (Begriff) under which all plant forms might be brought together. Though the type concept is implicit in his essay of 1790, the emphasis there is on the appendages, rather than on the plant as a whole, and he did not explicitly refer to the Urpflanze until 1817. He says, in his botanical autobiography, that this concept hovered before him in sensible form, though he recognised that it was actually supersensible;2 elsewhere he calls it, "eine symbolische Pflanze".3 If we translate Urpflanze as 'primitive plant',or 'primaeval plant', we are reading into it an evolutionary meaning which would have been foreign to Goethe's mind. To him the Urpflanze was a concept, from which the concepts of existing plant forms could be derived mentally; it carried no phylogenetic implications, and did not to him suggest any notion of an ancestral stock.

Similar ideas to those of Goethe were reached independently by his contemporary, Joseph-François Corréa da Serra (1750-1823), a botanist whose name seems to have passed into undeserved oblivion. Corréa da Serra was Portuguese by birth. He received a good education, and in youth he studied in Italy and sojourned for a long time in Rome. Being recalled to Portugal, he was instrumental in founding the Academy of Siences in Lisbon. In 1786 he was so unfortunate as to be denounced by the Inquisition, but he found an asylum in France. Later he was able to return to his own country, but other troubles followed, and he was obliged to seek refuge in England, where he was received cordially by Sir Joseph Banks and the Royal Society. It may be recalled that it was he who brought Robert Brown to Banks's notice, and initiated the friendship and collaboration with Banks which became Brown's mainstay. Towards the end of Corréa's life, he travelled and lectured in America. His experience of men and countries was thus wide and varied. D'Almeida,4 who is the authority for the facts of his life, points out that Corréa da Serra's existence was 'trop orageuse' to allow of any large-scale work; it may be, indeed, that his stormy career was responsible for the apparent 'paresse insouciante', of which de Candolle accuses him.5 The little that he published was, however, of an original character, and disclosed luminous ideas. Though his theoretical views were set forth merely by the way, in the course of small taxonomic and descriptive papers, they were of the most far-seeing kind, and expressed with a vivid literary sense, which makes one realise how it came about that his conversational mots delighted Paris; one which has survived is his description of the bizarre vegetation of New Holland as "Flore au bal masqué". He was just the man to throw out hints and suggestions, which more pedestrian minds could develop in detail. He affected the stream of morphological thought mainly through the influence which he exerted on A. P. de Candolle. The principal idea which de Candolle took from him was that of the plan underlying each group-a plan "suivi avec tenacité, mais varié avec richesse",6 and enforced with "stubborn versatility".7 This idea is echoed in de Candolle's Théorie Élémentaire de la Botanique, which is not a mere text-book, but an attempt to expound in their generality the logical principles which should serve as the basis for the study of organised beings. In this work de Candolle reached the point of centring his morphological interpretations in the concept of 'types primitifs et reguliers', of which he regarded all existing irregular forms as modifications.8 At a later stage he elaborated this idea, stating that "chaque famille de plantes, comme chaque classe de cristaux, peut être répresentée par un état régulier, tantot visible par les yeux, tantot concevable par l'intelligence; c'est ce que j'appelle son type".9 Though neither Goethe, Corréa da Serra, nor de Candolle appear to have realised the fact, the roots of this conception can be traced back as far as Aristotle., who points out that, within any given group of animals, "the parts are identical save only for a difference in the way of excess or defect".10 Aristotle's expression -"excess or defect"-implies deviations from an intellectually conceived type form, which is the norm or standard. His hint, pregnant as it was, remained undeveloped by later thinkers, and it is to the independent advocacy of Goethe and de Candolle that the type concept owed the part which it played in the morphology of the nineteenth century, and its still more important role in the modern revival of this branch of botany at the hands of Wilhelm Troll and his pupils.

Reviewing the type theory as expounded by Goethe, we see that he treated various issues together, which, for lucidity, it is well to keep apart. Within the plant itself, he postulated a single type for the different lateral appendages of the stem (e.g. foliage-leaves and flower parts) and other types of higher orders for different kinds of shoot (e.g. leafy shoots and flowers; or expanded inflorescences and capitula). Turning to the plant as a whole, he hinted at the concept of a type for each family of plants-an idea which de Candolle developed in full, on the background of his wide taxonomic knowledge. Finally Goethe postulated an archetype for all flowering plants-the Urpflanze. When we attempt to consider the archetypal plant, we are at once confronted with the main difficulty in the employment of the type concept, namely, that the mind has an almost irresistible propensity to transfer it from the plane of abstractions, where it belongs, to that of sensuous thinking-the plane of the visible and tangible. Goethe definitely regarded his archetypal plant as a supersensible conception, but he perhaps hardly realised how easily one might slip into the error of thinking about it pictorially, while believing oneself to be approaching it abstractly. This difficulty came to the fore in a certain historic botanical discussion which he held with Schiller.11 Goethe tells us that he demonstrated his theory of the metamorphosis of plants to Schiller in vivid fashion, and that, with several characteristic strokes of the pen, he caused the symbolic plant to arise before his eyes; but Schiller shook his head, and said, "This is not an experience (keine Erfahrung); it is an idea." Schiller's implication, that he was confusing abstract thought with sensuous perception, was, perhaps, scarcely fair to Goethe, since he was, in fact, feeling his way, even if half unconsciously, to a mode of contemplative thought in which both these activities should be synthesised and transcended.12 However, the criticism, even if it did not find its target in Goethe, was richly deserved by some of the lesser men who followed in his tracks. Turpin, for instance, attempted in 1804 an elaborate portrait of the archetypal flowering plant; at that date he had not read Goethe's Metamorphose, but, in a book more than thirty years later,13 he published his picture as an appropriate illustration of Goethe's ideas. It appeared as an engraving, the large size of which made it possible to represent the "Végétal type, idéal, appendiculé", as more than 19 inches high; and this space is needed, for every variety of incompatible detail is crowded into it. The unity of the plant is preserved only by its possession of one main axis. This axis bears a whole series of cotyledons of divers shapes, which are succeeded by a bewildering assortment of leaves of every kind-simple and compound, tendrillar, bulbil-bearing, and rooting-associated with a compendium of axillary branches, including such unusual forms as a fertile cladode. A gradation of members from foliage-leaves to stamens is shown in connexion with a terminal flower. This flower, and another on a lateral branch, obligingly offer examples of different forms of gynaeceum, and also show individual anthers dehiscing by distinct mechanisms. The whole thing is a botanist's nightmare, in which features, which could not possibly coexist, are forced into the crudest juxtaposition.

Long after Turpin constructed his picture, another representation of the Urpflanze was published by Schleiden.14 It has the merit of being simpler than that of Turpin, but it belies Goethe's conception of the floral members, and would, one feels, have been almost as distressing to him as Turpin's effort. The absurdity of these particular pictures is so patent that it becomes harmless, but the same inclination to give the archetype visible and tangible expression, took, at a later date, a subtler and hence more insidious form.

In the period that opened with the publication of The Origin of Species, the scientific world became convinced, both that evolution had taken place, and also that the natural selection of chance variations provided a master key to the understanding of the process.15 Up to that time plant forms had been considered worthy of study in and for themselves, and where relations between these forms were recognised, this relation was treated as logical rather than temporal. In the Darwinian reorientation of biology, however, the attention of most botanists was diverted from pure morphology to the use of form data in support of speculations about evolution. This was particularly so where flowering plants were concerned, since the most direct kind of evidence, that of the geological record, was rarely available. To evolutionary schemes, the type concept fell an immediate victim.16 The Darwinian school seized upon Goethe's archetypal flowering plant, and the notion, common to him and de Candolle, of a minor archetype for each family; detached these ideas from their context in the world of thought; set them up in the world of experience; and assumed their actual historic existence. Goethe's conception of the Urpflanze, which in his mind had a timeless quality, was thus transferred to some specific period of the past, as the Ancestral plant; and it was imaged as something which would have been visible and tangible, if mankind had been there to see and handle. In the intellectual atmosphere of the later nineteenth century, this forcing of Goethe's ideas into an evolutionary frame, seemed a perfectly natural proceeding. To many workers at that time, the diversion of biology into historical channels was a welcome relief; since it transformed theoretical botany into something material, amenable to picture-thinking, and not demanding difficult mental activity of a metaphysical kind. Thus, by a feat of legerdemain, which seems to have passed almost unnoticed, the Ancestral Plant was substituted for the Archetypal Plant, and those characters which had, with reason, been attributed to the mental conception of the archetype, were, without further justification, assumed to have been proven for an actual, historically existent ancestor. In justice it should, however, be recalled that T. H. Huxley17-deeply imbued though he was with Darwinian ideas-had the candour to admit that the existence of morphological relations between species was not actually incompatible with the doctrine of Special Creation, as expounded in Genesis. Such recognition of the independence in thought of morphology and phylogeny-of the logical and the genealogical, to use the terms of a recent French writer18-was, however, rare. The abstract nature of the archetypal plant was glossed over by most biologists, and, if a series could be constructed in the mind, leading from one kind of plant to another, it was considered legitimate to endow the intermediate mental constructs with existence in time and space. It was assumed that one organism was the descendant of another, when it was proven, or even merely surmised, that it was subsequent to that other-the same fallacy that, in argument, identifies propter hoc with post hoc. This confusion lent facility to the tracing of phylogenies, a pursuit which, for a long time, fascinated and obsessed morphological thought. The manufacture of these pedigrees was greatly simplified by the assumption that the progress of evolution on all the main lines was necessarily from the simple to the complex, so that the series studied were regarded, without any doubt, as having an irreversible direction in this sense; indeed, in the animal world, evolution towards man as a climax was assumed as too self-evident to need any kind of proof. It is curious that this assumption should have been made so lightly, for it appears that, to Plato, it was the opposite hypothesis that seemed the obvious one. According to the myth of creation in the Timaeus, birds, land animals, fishes, shell-fish, and so on, do not represent progressive series proceeding upwards to the human type, but are, on the contrary, degradations from that type.19 The land animals, for instance, are imaged as having come from men, who, having no use for philosophy, had their forelimbs and heads drawn down to earth.20 This shows that the idea of a progression in living things from relative simplicity to relative complexity has not always been regarded as an inescapable principle; but it seemed so to nineteenth century biologists, and it ratified their faith in the compilation of schemes of botanical ancestry. No doubt as to the factual existence of these ancestries was allowed to intrude, and the only serious differences of opinion arose out of the question as to whether ontogeny, or teratology, or comparison of mature forms, or other branches of study, provided the most reliable clues to the unravelling of pedigrees. In other words, the existence of a phylogenetic tree was regarded as 'given'; and it only remained to discuss what particular evidence best revealed its ramifications. Gradually, however, the facile Darwinian view-so easy to understand, and therefore so fatally easy to accept-lost its hold, and morphologists began to question, not merely what evidence might be used, but even whether the problem of phylogenetics itself was a genuine or an imaginary one. It was clear that structural series could be made out among living things, but were these series temporal or merely logical? As far as flowering plants were concerned, phylogenetic schemes had been based on the assumption that there was an actual historic 'tree', starting from a single original stock, and that there was no reason why the course of the branches up to the ultimate twigs should not eventually be exposed to view; but the possibility had at last to be faced that no such tree might ever have existed, and that morphological series might be merely mental constructs with no validity in time. This full scepticism was reached slowly and with much hesitation. The first step was the postulation of two or several original stocks, instead of the single one, but gradually polyphylesis1 on a much more extensive scale suggested itself, until, first a bush,21 and at last a sheaf, rather than a tree, came to seem an appropriate metaphor. In any case, the tree analogy cannot be pushed very far, for phylogenetic trees have the peculiarity of being rootless.23 Finally, as taxonomic and morphological work disclosed vista after vista of complexity, these simple images of tree and sheaf were replaced by the picture of a tangled reticulum in more than two dimensions. This was not altogether a new idea; it may be regarded as an outcome of an opinion expressed in 1810 by Robert Brown, who wrote of Nature herself connecting groups of organisms net-wise rather than chain-wise.24 This view of the matter has no obvious relation to the orthodox theory of descent with modification, and its complexity converts into a groping pursuit of will-o'-the-wisp, the attempt to deduce the characters of synthetic ancestral stocks. The reticulum picture thus involves a loss of faith in phylogenetics, but this loss has been a loss of shackles, and has given freedom for a revival of comparative morphology, studied for its own sake, and not in subservience to any evolutionary theory. This change of outlook has increased rather than diminished the interest of typology-indeed the modern German school of morphology is disposed to take "Back to Goethe" as its motto. The change has, however, demanded a reassessment of our scheme of thought; we have to decide what exact significance in this reorientation is to be retained by the 'type' concept.25

In this connexion it is helpful to consider the views of certain pre-Darwinian writers. In 1768, long before a belief in evolution had become part of the biologist's creed, Robinet26 published a book in which he advocated the idea that all beings-passing upwards from the stone, through plant, insect, reptile, and quadruped, to the most excellent form of Being, which is 'la forme humaine'-are 'conçus et formés' after a 'model' or 'original example', which he calls the 'prototype'. He was thus, in a certain sense, foreshadowing the type concept of later writers. He illustrates his views by pointing out that a cave dwelling, a savage's hut, a shepherd's cot, an ordinary house, and a palace, may all be considered as graduated variations upon one architectural plan. None of the humbler forms claims to be an Escurial or a Louvre, but they all hark back to the same 'dessein primitif' as the most magnificent palace, since every kind of dwelling is a product of one and the same basic idea, developed to a greater or lesser extent. The intensity of Robinet's desire to find transitional connecting links of the Chain of Being, led him into some extravagant beliefs; he refers, for instance, in all seriousness to a merman who was fished out of the sea on the Suffolk coast in A.D. 1187. This unfortunate credulity coexisted in his mind, however, with a capacity for real insight. He used the word 'evolution' in describing such relations as those between the different kinds of human dwelling, but this term did not mean to him an historic movement, progressing through time, such as the phylogeneticist visualises. He has, on the contrary, the merit of considering form in itself-not as subordinate to time, but sub specie aeternitatis. It is to this standpoint that morphology has to readjust itself, if the study of form is to reveal its full significance; there must be a return to considering the type as a purely abstract intellectual concept, bearing the same relation to the individuals making up the group as the concept 'man' (using the word as a 'universal' in the philosopher's sense) bears to the aggregate of individual men. The types or 'primal patterns', as Owen27 called them, thus have much in common with certain aspects of the Platonic 'forms'. In the comparative study of morphology, the idea of the type offers a fixed centre from which to appraise the structural variations which occur within every group. In 1840, long after Robinet's time, but before the tide of belief in evolution had set in, Whewell had expressed this thought in a peculiarly luminous way. He had the insight to realise that a natural class of objects "is determined, not by a boundary line without, but by a central point within; not by what it strictly excludes, but by what it eminently includes; by an example, not by a precept; in short, instead of Definition, we have a Type for our director".28 In this passage, Whewell "builded better than he knew", for his further discussion shows that he did not, in fact, rise to an abstract conception of the type itself, but that he identified the type of a genus, for instance, with the species actually existing which showed the most typical characters. Despite the aperçu just quoted, he had not become fully aware that the type concept is essentially mental-an intellectual instrument wherewith the mind brings order into the variegated manifold of phenomena.

When we turn from these broader questions to the individual plant and its components, we must consider what, in the eyes of the modern botanist, should be the status of Goethe's type appendage, which he called the leaf (Blatt), but for which the generalised term, phyllome, is preferable. Like the archetypal plant, the type phyllome is a valid concept so long as it is kept consistently on the abstract plane, but this has not always been done. Goethe's disciples have been too apt to speak as though the sepal and the carpel, for instance, owed their derivation to the foliage-leaf: even a botanist so sagacious as Robert Brown, wrote in 1822 of "the Leaf, from whose modifications all the parts of the flower seem to be formed",29 while, in the twentieth century, Troll slips into the misconception of calling these parts, "Umbildungsformen von Laubblättern".30 Goethe himself, however, did not fall into this error, nor did the most discriminating of his followers. Asa Gray, for instance, made it clear that it was a mistake to suppose that petals, stamens, and carpels had existed previously in the state of foliage,31 and he held that the expression, 'metamorphosis', ought to be used in a purely figurative sense.32 Goethe, indeed, always insisted on relating the phenomena of metamorphosis to one another in both directions. He held that it was as legitimate to call a foliage-leaf an expanded sepal, as to call a sepal a contracted foliage-leaf; this reversibility obviously precludes the idea of historic derivation.33 The metamorphosis theory, as Goethe himself understood it, thus means that the generally recognised relationship between the different appendicular members arises out of the fact that they are all manifestations of one type-phyllome, non-historic in character. This idea has a number of obvious advantages as a working hypothesis; but the question remains whether it is to be received as an ultimate dictum, or whether, in present-day thought, some further and more satisfying generalisation can be developed out of it.34 We cannot come to any critical conclusion on this point if we con-centrate attention exclusively on the phyllome itself; the problem is a wide one, and in the next chapter we will treat it on a less restricted background.

1By permission of the Editor of Biological Reviews, parts of Arber, A. (1937a) have been incorporated in this chapter and elsewhere.
2Troll, W. (1926), p. 202.
3 Troll, W. (1926) (Goethe, J. W. von, Glüchliches Ereignis, p. 267).
4lmeida, F. d' (1824); see also Candolle, A. P. de (1862), pp. 162-4.
5Candolle, A. P. de (1869.), p. 414.
6Corréa da Serra, J. F. (1806), p. 62.
7Corréa da Serra, J. F. (1796), p. 494.
8Candolle, A. P. de (1813), p. 144.
9Candolle, A. P. de (1827), vol. II, p. 241.
10Thompson, D'Arcy W. (1910), vol. iv [Hist. animal. I. l. 486a. (Oxf. trans.)].
11Troll, W. (1926) [Goethe, J. W. von, Glückliches Ereignis, p. 267].
12See pp. 208-11.
13Turpin, P. J. F. (1837), pl. 3.
14Schleiden, M. J. (1848), pl. 4, and p. 83.
15See Robson, G. C. and Richards, O. W. (1936) for a critical, but cautious and sympathetic, revaluation of the selection hypothesis.
16According to Meyer, A. (1934), pp. 128 -30, in zoology, the transformation typology into phylogenetics can be witnessed in C. Gegennbaur's work; cf. the (1859, pre-Darwinian) edition of his Grundzüg der Vergleichenden Anatomie, with the second (1870) edition.
17Huxley, T. H. (1888), p. 112.
18Hocquette, M. (1946), p. 121.
19It should be noted that the word archetype is not used by Plato, but can be traced back to Philo Judaeus (f/. ante A.D. 40); see Wolfson, H. A. (1947), vol. I, p. 238.
20Cornford, F. M. (1937), p. 358.
21For early references to polyphylesis see Celakovsky, L. J. (1878), footnote, p. 154. See also Huxley, T. H. (1888), pp. 122-3. For an account of H. Przibram's zoological work, in which the idea of polyphylesis reaches its extremest expression, see Caiman, W. T. (1930), pp. 89-90.
22The 'bush' analogy was suggested by Sachs long ago (cf. Pringsheim, E. G.(1932), p. 146); it has been emphasised recently by other writers, e.g. Vialleton, L. (1930), p. 373.
23Hayata, B. (1931), p. 332.
24Brown, R. (1810), vol. i. Praemonenda: "ipsa natura enim, corpora organica reticulatim potius quam catinatim connectens".
25The relation of evolutionary and typological morphology is discussed in Thomas, H. Hamshaw (1947) from the standpoint of a phylogeneticist.
26Robinet, J. B. R. (1768a or 1768b); for a study of Robinet's idea of the type,see Lovejoy, A. O. (1936), especially pp. 278-80.
27Owen, R. (1894), vol. I, pp. 387-8
28Whewell, W. (1840), vol. I, p. 476.
29Brown, R. (1822), footnote, p. P.l 1.
30Troll, W. (1935, etc.), vol. I, p. 36.
31Gray, A. (1858), p. 291.
32Gray, A. (1887), p. 169.
33Cf. p. 159.
34Cf. pp. 85, 86, 159-161.