Evolution Of Plant Classification
( Originally Published 1912 )
IN the earliest European works on natural history—those of the Aristotelian school—we meet with an attempt to classify the different varieties of plants. It was inevitable that the writers of this school should make such an attempt, since no mind trained in Greek philosophy could be content to leave a science in the condition of a mere chaos of isolated descriptions. At first the most obvious distinction, that of size, was used as the chief criterion whereby to separate the different groups of the vegetable kingdom. In the ` History of Plants' of Theophrastus, we find Trees, Shrubs, Bushes and Herbs treated as definite classes, within which, cultivated and wild plants are distinguished. Other distinctions of lower value are made between evergreen and deciduous, fruiting and fruitless, and flowering and flowerless plants.
Albertus Magnus, who kept alive in the Middle Ages the spirit of Aristotelian botany, was more advanced than Theophrastus in his method of classification. It is true that he divides the vegetable world into Trees, Shrubs, Undershrubs, Bushes, Herbs and Fungi, but at the same time he points out that this is an arbitrary scheme, since these groups cannot always be distinguished from one another, and also because the same plant may belong to different classes at different periods of its life. A study of the writings of Albertus reveals the fact that he had in mind, though he did not clearly state it, a much more highly evolved system, which may be diagrammatically represented as follows. The modern equivalents of his different groups are shown in square brackets :
I. Leafless plants [Cryptogams in part].
II. Leafy plants [Phanerogams and certain Cryptogams].
1. Corticate plants [Monocotyledons].
2. Tunicate plants [Dicotyledons].
The word tunicate in the above table is used for the plants which Albertus describes as growing " ex ligneis tunicis." It seems clear from this expression that he realised that there was an anatomical distinction between Dicotyledons and Monocotyledons.
Considering how much Albertus had achieved, it is somewhat curious that Cesalpino, who represented Aristotelian botany in the sixteenth, as Albertus did in the thirteenth century, should have produced so inadequate a system as his own contribution to the subject. We owe to him one marked advance, the recognition, namely, of the importance of the seed. On the whole, however, his classification savours too much of having been thought out in the study, and it suffers by comparison with other systems of about the same period, such as those of de l'Obel and Bauhin, which were arrived at rather by instinct, acting upon observation, than by a definite and self-conscious intellectual effort.
Cesalpino makes his main distinction, on the old Aristotelian plan, between Trees and Shrubs on the one hand, and Undershrubs and Herbs on the other. He divides the first of these groups into two, and the second into thirteen classes, depending chiefly on seed and fruit characters. Very few of these classes really represent natural groups, and the chief of all distinctions among Flowering Plants, that between Dicotyledons and Mono-cotyledons, which was foreshadowed by Albertus, is almost lost to sight.
When we turn from the botanical philosophers to the herbalists proper, we find an altogether different state of affairs. The Aristotelian botanists were conscious, from the beginning, of the philosophic necessity for some form of classification. The medical botanists, on the other hand, were only interested in plants as individuals, and were driven to classify them merely because some sort of arrangement was necessary for convenience in dealing with a large number of kinds. The first Materia Medica, that of Dioscorides, shows some attempt at order, but the arrangement is seldom at all natural. Occasionally the author groups together plants which are nearly related, as when he treats of a number of Labiates, or of Umbellifers successively—but this is rare.
Pliny was not, strictly speaking, a medical botanist, but at the same time he may be mentioned in this connection, since his interest in plants was essentially utilitarian. Like Theophrastus, he begins his account of plants with the trees, but his reason for so doing is profoundly different from that of the Greek writer, and illustrates the divergence between what we may call the anthropocentric and the scientific outlook upon the plant world. Theophrastus placed trees at the head of the vegetable kingdom, because he considered their organisation the highest, and most completely expressive of plant nature ; Pliny, on the other hand, began with trees because of their great value and importance to man. As an example of his ideas of arrangement, we may mention that he places the Myrtle and Laurel side by side, because the Laurel takes a corresponding place in triumphs to that accorded to the Myrtle in ovations !
Turning to the herbals themselves, we find that the earliest show no trace of a natural grouping, the plants being, as a rule, arranged alphabetically. This is the case, for instance, in the Latin and German Herbarius, the Ortus Sanitatis and their derivatives, and even in the herbals of Brunfels and of Fuchs in the sixteenth century. In Bock's herbal, on the other hand, the plants are grouped as herbs, shrubs and trees, according to the classical scheme. The author evidently made some effort, within these classes, to arrange them according to their relationships. In the preface to the third edition he writes—" I have placed together, yet kept distinct, all plants which are related and connected, or otherwise resemble one another and are compared, and have given up the former old rule or arrangement according to the A.B.C. which is seen in the old herbals. For the arrangement of plants by the A.B.C. occasions much disparity and error."
Although the larger classificatory divisions, as now understood, were not recognised by these early workers, they had at least a dim understanding of the distinction between genera and species. This dates back to Theophrastus, who showed, by grouping together different species of oaks, figs, etc., that he had some conception of a genus. We owe to Konrad Gesner the first formulation of the idea that genera should be denoted by substantive names. He was probably the earliest botanist who clearly expounded the distinction between a genus and a species. In one of his letters he writes—"And we may hold this for certain, that there are scarcely any plants that constitute a genus which may not be divided into two or more species. The ancients describe one species of Gentian; I know of ten or more."
Very little of Gesner's botanical work was ever published, and it was left to Fabio Colonna to put before the botanical world the true nature of genera. He held most enlightened views on the subject, and, in 1616, clearly stated in his Ekphrasis ' that genera should not be based on similarities of leaf form, since the affinities of plants are indicated not by the leaf, but by the characters of the flower, the receptacle, and, especially, the seed. He brought forward instances to show that previous authors had sometimes placed a plant in the wrong genus, because they only attended to the leaves and ignored the structure of the flower.
In the writings of Gaspard Bauhin, at the end of the sixteenth and the beginning of the seventeenth century, the binary system of nomenclature is used with a high degree of consistency, each species bearing a generic and specific name, though sometimes a third, or even a fourth, descriptive word is added. These extra words are not, however, really essential. In the preface to the ` Phytopinax' (i 596) Bauhin states that, for the sake of clearness, he has applied one name to each plant and added also some easily recognisable character'.
The binomial method was foreshadowed at a very early date, for in a fifteenth-century manuscript of the old herbal `Circa instans,' to which we have referred on p. 24, this system prevails to a remarkable extent.
When we turn to those general schemes of classification which were evolved by the herbalists of the sixteenth century, we are at once struck by the great difference existing between the principles on which these schemes are based, and those at which we have arrived at the present day. To classify plants according to their uses and medicinal properties is obviously the first suggestion that arises, when the universe is regarded from a simple, anthropocentric standpoint. In the Grete Herball of 1526 we get a ludicrously clear example of this method, applied to the special case of the Fungi. " Fungi ben mussherons. ...There be two maners of them, one maner is deedly and sleeth [slayeth] them that eateth of them and be called tode stoles, and the other Booth not." . This account of the Fungi occurs also in the earlier manuscript herbal, `Circa instans,' mentioned in the last paragraph.
This theory of classification has been shown in more recent times to contain the germ of something more nearly approaching a natural system than one would imagine at first sight. Both Linnæus and de Jussieu have pointed out that related plants have similar properties, and, in 1804, A. P. de Candolle, in his 'Essai sur les propriétés médicales des Plantes, comparées avec leurs formes extérieures et leur classification naturelle,' carried the argument much further. He showed that in no less than twenty-one families of flowering plants, the same medicinal properties were found throughout all the members of the order. This is very remarkable, when we remember that the state of knowledge at that time was such that de Candolle was obliged to dismiss a large number of orders with the words "properties unknown." Quite recently the subject of the differentiation of groups of plants according to their chemistry has again come to the fore, and, in the future, chemical characters will probably be numbered among the recognised criteria for use in elaborating schemes of classification.
In the history of botanical classification, the first advance from the purely utilitarian standpoint was marked by the recognition of the fact that the structure and mode of life of the plants themselves are of importance. In the work of writers such as Dodoens and d'Aléchamps, to take two typical examples, we find the issues curiously confused by the working of three different principles side by side ; that is to say, by the simultaneous insistence (i) on the habitat, (ii) on the "virtues," and (iii) on the structure, as affording clues to the systematic position of the plant in question. The herbalist thus erects his scheme on a basis consisting of a confused medley of ecological, medical, and morphological principles. An enumeration of the eighteen headings, under which d'Aléchamps, in 1586, described the vegetable kingdom, so far as it was then known, will show the perplexities which surrounded the first gropings after a natural system. His headings may be translated as follows :
I. Of trees which grow wild in woods.
Among these eighteen groups, the only ones which have any pretension to being natural are V I (Umbellifers) and XI V (Thistles), and these merely approximate roughly to related groups of genera. Among the Umbellifers we meet with Achillea and other genera which do not really belong to the order, whilst, with the Thistles, there are grouped other spiny plants, such as Astragalus tragacantha, which, in a natural system, would occupy a place remote from the Composites.
In spite of the fact that improved systems of classification, to which we shall shortly refer, were put forward in the sixteenth and early seventeenth centuries, we find that, as late as 164o, John Parkinson in his well known herbal, divided all the plants then known into seventeen classes or tribes—the sequence in which these classes were placed having, in most cases, no meaning at all. A few of his tribes are natural, but many are valueless as an expression of affinities. As an example we may mention his third class, "Venemous, Sleepy, and Hurtfull Plants, and their Counterpoysons," and his seventeenth, Strange and Outlandish Plants." In Parkinson's classification, we see Botany reverting once more to the position of a mere handmaid to Medicine.
In the first book of Dodoens" Pemptades' (1583) the principles of botany are discussed. The old Aristotelian classification into Trees, Shrubs, Undershrubs and Herbs is accepted, but with some reservations. The author points out that an individual plant may, owing to cultivation, or from some other cause, pass from one class into another. He instances Ricinus, which is an herbaceous annual with us, but a tree in other countries'.
The general scheme of classification, which Dodoens propounded, has much in common with that of d'Aléchamps, which we have already outlined. Within the larger groups, he shows a stronger perception of natural grouping than appears in his arrangement of the larger classes themselves. He often grouped together genera which we now regard as members of the same natural order, and species which we now look upon as belonging to a single genus. For instance he brought together genera belonging respectively to the Geraniaceæ, Hypericaceae, Plantaginaceae, Crucifer e, Compositæ, etc. In some cases, however, he was only partially successful, as in the Umbelliferæ, among which he described Nigel a (Love-in-a-Mist) and a couple of Saxifrages. This example shows how little stress was laid on the flowers and fruit at this time, from the point of view of classification. The general habit, and the shape of the leaves were the features that received most attention.
Resemblances and differences between the forms of the leaves alone must naturally appear to the botanist of the present day to be a very inadequate basis for a general system of classification. Nevertheless Mathias de l'Obel worked out a scheme on these lines which had great merit, and was a considerable advance on previous efforts. He put forward his system in his ` Stirpium adversaria' (1570 -71) and used it also in his later work. It was thus published much earlier than the very primitive schemes of d'Aléchamps and Dodoens to which we have just referred. The best point of his system is that, by reason of their characteristic differences of leaf structure, he distinguishes the classes now known to us as Monocotyledons and Dicotyledons. He introduces a useful feature in the shape of a synoptic table of species which precedes each more or less natural group of plants. The superiority of his classification to the other arrangements in the field at the time was immediately realised. We have evidence of this in the fact that, after his ` Kruydtbceck' was published, Plantin brought out an album of the wood-engravings used in the book, which, although they had also appeared as illustrations to the works of Dodoens and de l' Écluse, were now arranged as in the scheme put forward by de l'Obel, " according to their genus and mutual relationships."
There seems little doubt that de l'Obel made a more conscious effort than any of his predecessors to arrive at a natural classification, and that he realised that such a classification would reveal a unity in all living beings. In the preface to his ` Stirpium adversaria nova' of t 570 he writes—" For thus in an order, than which nothing more beautiful exists in the heavens or in the mind of a wise man, things which are far and widely different become, as it were, one thing."
De I'Obel's scheme is not expressed in the clear manner to which we have become accustomed in more modern systems, because, in common with other botanists of his time, he did not, as a rule, give names to the groups which we now call orders, or draw any sharp line of distinction between them.
De l'Obel's arrangement, in spite of its good features, had serious drawbacks. The anomalous Monocotyledons, such as Arum, Tamus, Aloe and Ruscus, are scattered among the Dicotyledons, while Drosera (the Sundew) appears among the Ferns, and so on. Similarities of leaf form, which are now regarded merely as instances of "homo-plastic convergence," are responsible for many curious groupings. For instance in the ` Kruydtbceck' we find the Twayblade (Listera), the May Lily (Maianthemum) and the Plantain (Plantago) described in succession, while, in another part of the book, various Clovers (Trifolium), Wood Sorrel (Oxalis) and Anemone hepatica are grouped together. It is also not surprising that the Marsh Mari-gold (Caltha), the Waterlilies (Nymphaea and Nuphar), Limnanthemum and Frogbit (Hydrocharis) should follow one another, or that de l'Obel should have brought together the Broomrape (Orobanche), the Toothwort (Lathræa), the Bird's-nest Orchid (Neoltia) and a number of Fungi. In these latter instances the author has really arrived at genuine biological (though not morphological) groups. He has recognised, on the one hand, the marked uniformity of the type of leaf characteristic of " swimming " water-plants, and, on the other hand, he has observed the leaflessness and absence of green colour, which are negative features common to so many saprophytes and parasites.
The perception of natural affinities among plants which, in the sixteenth and seventeenth centuries, was gradually, in a dim, instinctive fashion, arising in men's minds, is perhaps best expressed in the work of Gaspard Bauhin, especially in his ` Pinax theatri botanici' (1623). This work is divided into twelve books, each book being further sub-divided into sections, comprehending a variable number of genera. Neither the books nor the sections have, as a rule, any general heading, but there are certain exceptions. For instance, Book 11 is called `de Bulbosis,' and a section of Book Iv, including eighteen genera, is headed ` Umbelliferæ.' Some of the sections represent truly natural groups. Book III, Section vi, for example, consists of ten genera of Compositae, while Book III, Section II includes six Crucifers. Other sections contain plants of more than one family, but yet show a distinct feeling for relationship. For instance, Book v, Section I includes Solanum, Mandragora, Hyoscyamus, Nicoliana, Papaver, Hypecoum and Argemone—that is to say four genera from the Solanaceæ followed by three from the Papaveraceæ. The common character which brings them together here is, no doubt, their narcotic property, but, although no definite line was drawn between the plants belonging to these two widely sundered families, the order in which they are described shows that their distinctness was recognised. Some of Bauhin's other groups, however, which, like that just discussed, are distinguished by their properties, or, in other words, by their chemical features, have no pretension to naturalness from a morphological standpoint. This is the case with the group described in Book xi, Section III under the name of "Aromata," which consists of a heterogeneous assemblage of genera belonging to different orders, which are only connected by the fact that they all yield spices useful to man.
There is no doubt that, on the whole, Bauhin was markedly successful in recognising affinities within small cycles, but he broke down on the broader question of the relationships between the groups of genera so constituted. This is, however, hardly surprising when we remember how much difference of opinion exists among systematic botanists, even today, upon the subject of the relations of the orders to one another.
Like de l'Obel, Bauhin seems to have believed in the general principle of a progression from the simpler to the more highly developed forms. His application of this principle led him to begin with the Grasses and to conclude with the Trees. The question as to which groups among the Flowering Plants [Angiosperms] are to be considered as relatively primitive, is still, at the present day, an open one, but it would be generally conceded that Bauhin's arrangement cannot be accepted. There is little doubt, from the standpoint of modern botany, that the Grasses are a highly specialised group, while the " tree habit " has been adopted independently by many plants belonging to entirely different cycles of affinity, and thus, except in rare cases, it cannot be used as a criterion of relationship.
On the subject of the relations of the Cryptogams (flowerless plants) to the Phanerogams (flowering plants), Bauhin had evidently no clear ideas, but such could hardly be hoped for in the state of knowledge of that time. We find, for instance, the Ferns, Mosses, Corals(!), Fungi, Alga, the Sundew, etc., sandwiched between some Leguminosa, and a section consisting chiefly of Thistles.
The classification put forward by the Bohemian botanist, Zaluziansky, in 1592, although in its general features no better than that of Dodoens, or of d'Aléchamps, and certainly less satisfactory than that of de l'Obel or the later scheme of Bauhin, is an improvement on all of these in one particular, namely, that he begins with the Fungi and deals next with Mosses. After the Mosses he describes the Grasses, and his classification concludes with the Trees. He was thus evidently attempting to pass from the simpler to the more complex, and his arrangement indicates that, unlike certain other botanists of his time, he looked upon the Lower Cryptogams as comparatively simple and primitive plants. He was not so clear-sighted, however, on the subject of the Ferns, for he placed them with the Umbelliferw and some Composite, no doubt because he was influenced by the form of the leaf.
It is curious that Cesalpino, who, as we have pointed out, had arrived at the very important principle that the seed and fruit characters were of major value in classification, yet put forward a system which was distinctly inferior to that of Gaspard Bauhin, although the latter appears to have been guided by no such general principles.
Probably the reason for this is to be sought in the fact that no system of classification can represent natural affinities, unless it takes into account the nature of the plant as a whole. It is true that, compared with the characters of the reproductive organs, the leaf-form and habit, owing to their plasticity, have to be used with great discretion as systematic criteria, but, nevertheless, no system of classification can afford to ignore them entirely. Cesalpino based his scheme too exclusively upon seed characters, to the neglect even of the structure of the flower, and, curiously enough, although he laid so much stress upon the nature of the seed, he did not grasp the fundamental distinction between the embryos of the Monocotyledons and the Dicotyledons, due to the possession of one, and two seed-leaves respectively. The chief drawback of his scheme, however, was his failure to realise that living organisms are too complex to fall into a classification based on any one feature, important as that feature may prove to be when used in conjunction with other characters.
Those herbalists, on the other hand, who attacked the - problem of the classification of plants without any pre-conceived, academic theory, depended, one might almost say, on the glimmerings of common sense for the recognition of affinities. This was no doubt a dim and fitful illumination, but it was at least less partial than the narrow, lime-light beam of a rigid theory.