In any attempt to estimate the advance which the science owes to the labours of Linnaeus, the chief prominence must be assigned to two points; first to his success in carrying out the binary nomenclature in connection with the careful and methodical study which he bestowed on the distinguishing of genera and species; this system of nomenclature he endeavoured to extend to the whole of the then known vegetable world, and thus descriptive botany in its narrower sense assumed through his instrumentality an entirely new form, which, serving as a model for the naming and defining of the larger groups, could be applied without modification to the founding and completing the natural system. When at a later time Jussieu and De Candolle marked out their families and groups of families, their mode of proceeding was in the main that of Linnaeus when distinguishing his genera by abstraction of specific differences. This merit has been always assigned to Linnaeus without reserve. The second merit has been less recognised, and yet it is at least of equal importance; it is that of having first perceived that the attempt made by Cesalpino and his successors to found a system, that shall do justice to natural affinities, on predetermined marks can never succeed. Linnaeus framed his artificial sexual system, but he exhibited a fragment of a natural system by its side, while he repeatedly declared that the chief task of botanists is to discover the natural system. Thus he cleared the ground for systematic botany. He made use of his own system, because it was extremely convenient for describing individual plants, but he ascribed all true scientific value exclusively to the natural system; and with what success he laboured to advance it may be gathered from the fact, that Bernard de Jussieu founded his improved series of families on the fragment of Linnaeus, and that his nephew, A. L. de Jussieu, by simply adopting Linnaeus’ conception of the principle which lies at the foundation of the natural system, succeeded in carrying it on to a further stage of development.
The main features of Linnaeus’ theoretical botany can best be learned from the ‘Philosophia Botanica,’ which may be regarded as a text-book of that which Linnaeus called botany, and which far surpasses all earlier compositions of the kind in perspicuity and precision, and in copiousness of material; and indeed it would be difficult to find in the ninety years after 1781 a text-book of botany which treats what was known on the subject at each period with equal clearness and completeness. In giving the reader some idea of the way in which Linnaeus deals with his subject, it will be well to pass over the first two chapters, which discuss the literature and the various systems which had been proposed, and turn to the third, which under the heading ‘Plantae’ treats of the general nature of plants, and specially of the organs of vegetation. The vegetable world, says Linnaeus, comprises seven families, Fungi, Algae, Mosses, Ferns, Grasses, Palms, and Plants. All are composed of three kinds of vessels, sap-vessels which convey the fluids, tubes which store up the sap in their cavities, and tracheae which take in air; these statements Linnaeus adopts from Malpighi and Grew. He gives no characteristic marks for the Fungi; of the Algae he says that in them root, leaf, and stem are all fused together; to the Mosses he ascribes an anther without a filament, and separate from the female flower which has no pistil; the seeds of the Mosses have no integument or cotyledons; this characteristic of the Mosses is explained in his paper entitled ‘Semina Muscorum’ in the ‘Amoenitates Academicae,’ ii. The Ferns are marked by the fructification on the under side of the fronds, which are therefore not conceived of as leaves. The very simple leaves, the jointed stalk, the ‘calyx glumosus,’ and the single seed mark the Grasses. The simple stem, the rosette of leaves at the summit, and the spathe of the inflorescence are characteristic of the Palms. All vegetable forms which do not belong to any of the previous families he names Plants. He rejects the customary division into herbs, shrubs, and trees as unscientific. This arrangement of the vegetable kingdom must not be confounded with Linnaeus’ fragment of a natural system, in which he adopts sixty-seven families (orders), the Fungi, Algae, Mosses, and Ferns forming each a family. He evidently introduces the divisions in the ‘Philosophia,’ in order that it may be seen how far the statements that follow are applicable to all the Vegetabilia or only to certain sections of them. The parts in the individual plant which the beginner must distinguish are three; the root, the herb[31], and the parts of fructification, in which enumeration Linnaeus departs from his predecessors, by whom the fructification and the herb together are opposed to the root. In the central part of the plant is the pith, enclosed by the wood which is formed from the bast; the bast is distinct from the rind, which again is covered by the epidermis; these anatomical facts are from Malpighi; the statement that the pith grows by extending itself and its envelopes is borrowed from Mariotte. Cesalpino’s view on the formation of the bud is expressed by Linnaeus in the statement, that the end of a thread of the pith passing through the rind is resolved into a bud, etc. The bud is a compressed stem, capable of unlimited extension till fructification puts a term to vegetation. The fructification is formed by the leaves uniting into a calyx, from which the apex of a branch issues as a flower about one year in advance, while the fruit arising from the substance of the pith cannot begin a new life till the woody substance of the stamens has been absorbed by the fluids of the pistil. In this way Linnaeus corrected Cesalpino’s theory of the flower, that he might take into account the sexual importance of the stamens discovered by Camerarius. He concludes by saying that there is no new creation but only a continuous generation, for which he gives the remarkable and thoroughly Cesalpinian reason, ‘cum corculum seminis constat parte radicis medullari.’
The root, which takes up the food, and produces the stem and the fructification, consists of pith, wood, bast, and rind, and is divided into the two parts, ‘caudex’ and ‘radicula.’ The ‘caudex’ answers pretty nearly to our primary root and rhizomes, the ‘radicula’ to what we now call secondary roots.
The herb springs from the root, and is terminated by the fructification; it consists of the stem, leaves, leaf-supports (‘fulcrum’), and the organs of hibernation (‘hibernaculum’). Then follow the further distinctions of stem and leaves; the terminology, still partly in use and resting essentially on the definitions of Jung, is here set forth in great detail. Linnaeus however does not mention the remarkable distinction between stem and leaf which Jung founded on relations of symmetry, and in general he shows less depth of conception than Jung, confining himself more to the direct impression on the senses, and so distinguishing sometimes where there is no real difference. Examples of this are furnished by the paragraph devoted to ‘fulcra.’ By this term he designates the subsidiary organs of plants, among which he reckons stipules, bracts, spines, thorns, tendrils, glands, and hairs. It appears from this, that Linnaeus did not extend the idea of the leaf (‘folium’) to stipules and bracts, and the examples he gives of tendrils show at the same time that he was ignorant of the different morphological character of the organ in Vitis and Pisum. The putting the seven organs above-named together under the idea of ‘fulcrum’ shows plainly enough that Linnaeus, in framing his terminology, aimed only at distinguishing what was different to the sense by fixed words, in order to obtain means for short diagnoses of species and genera. He had no thought of arriving at more general propositions from a comparison of forms in plants, in order to attain to a deeper insight into their nature. The same thing appears from his notion of ‘hibernaculum,’ by which he understands a part of the plant which envelopes the stem in its embryonal state and protects it from harm from without; he here distinguishes bulbs from the winter buds of woody plants. In this course of mixing up morphological and biological relations of organs he was followed by botanists till late into our own century.
Linnaeus goes far beyond his predecessors in distinguishing and naming the organs of fructification, the subject of the fourth chapter of the ‘Philosophia Botanica.’ The fructification, he says, is a temporary part in plants devoted to propagation, terminating the old and beginning the new. He distinguishes the following seven parts: (1) the calyx, which represents the rind, including in this term the involucre of the Umbelliferae, the spathe, the calyptra of Mosses, and even the volva of certain Fungi,—another instance of the way in which Linnaeus was guided by external appearance in his terminology of the parts of plants; (2) the corolla, which represents the inner rind (bast) of the plant; (3) the stamen, which produces the pollen; (4) the pistil, which is attached to the fruit and receives the pollen; here for the first time the ovary, style, and stigma are clearly distinguished. But next comes as a special organ (5) the pericarp, the ovary which contains the seed. As bulbs and buds were treated not simply as young shoots, but as separate organs, so here too the ripe fruit is regarded not merely as the developed ovary, but as a special organ. Nevertheless, Linnaeus distinguishes the different forms of fruit much better than his predecessors had done. (6) The seed is a part of the plant that falls off from it, the rudiment of a new plant, and it is excited to active life by the pollen. The treatment of the seed and its parts is the feeblest of all Linnaeus’ efforts; he follows Cesalpino, but his account of the parts of the seed is much more imperfect than that of Cesalpino and his successors. The embryo is called the ‘corculum,’ and two parts are distinguished in it, the ‘plumula’ and the ‘rostellum’ (radicle). The cotyledon is co-ordinated with the ‘corculum,’ and is regarded therefore not as part of the embryo but as a distinct organ of the seed; it is defined as ‘corpus laterale seminis bibulum caducum.’ Nothing could be worse, and it seems almost incredible that so bad a definition and distinction could be given in 1751, and again in 1770, by the first botanist of his time, when Malpighi and Grew, nearly a hundred years earlier, had illustrated the parts of the seed and even the history of its development and its germination by numerous figures. He does not mention the endosperm, evidently confounding it with the cotyledon, though Ray had already distinguished it clearly from the other parts of the seed. Linnaeus’ terminology of the seed supplies more than sufficient corroboration of our previous remark, that he shows incapacity for the careful investigation of any object at all difficult to observe, and it will now seem a small matter that he, like most of the earlier botanists, treats one-seeded indehiscent fruits as seeds, and hence makes the pappus a part of the seed. (7) By the word ‘receptaculum’ he understands everything by which the parts of the fructification are connected together, both the ‘receptaculum proprium,’ which unites the parts of the single flower, and the ‘receptaculum commune,’ under which term he comprises the most diverse forms of inflorescence (umbel, cyme, spadix).
He concludes with the remark that the essence of the flower consists in the anther and the stigma, that of the fruit in the seed, that of the fructification in the flower and the fruit, and that of all vegetable forms in the fructification, and he adds a long list of distinctions between the organs of fructification with their names; among these organs appear the nectaries, which he was the first to distinguish.
In the fifth chapter he discusses the question of difference of sex in plants. His views on this subject have been already mentioned in order to show that they were entirely founded on worthless scholastic deductions; here we may quote a few of the propositions which were famous in after times. We assume, he says, that two individuals of different sexes were created in the beginning of things in every kind of living creatures. Plants, though they are without sensation, yet live as do animals, for they have a beginning and an advance in age (aetas), and are liable to disease and death; they have also a power of movement, a natural appetency (propulsio), an anatomy, and an organic structure (organismus). Simple explanations are given of these words, but they prove nothing about the matter. He then expounds the whole theory of sexuality, which is made to rest entirely on scholastic arguments, and in doing this he spins out to excessive length the parallel which he draws between the conditions of sexuality in animals and plants. It is manifestly this chapter of the ‘Philosophia Botanica,’ together with the treatise ‘Sponsalia Plantarum,’ which led the adherents of Linnaeus, who were ignorant of the older literature of the subject and were much impressed by his scholastic dexterity, to celebrate him as the founder of the sexual theory of plants; whereas a more careful study of history shows incontrovertibly that Linnaeus helped in this way to disseminate the doctrine, but did absolutely nothing to establish it.
The writings of Linnaeus which we have hitherto examined are occupied with the nature of plants, and of this he knew nothing more than he gathered from the investigations and reflections of his predecessors; and it is here especially that his peculiar scholasticism is exhibited in contrast with the facts obtained by induction which he communicated to his readers. But the strong side of his intellect appears with splendid effect in the succeeding chapters of the ‘Philosophia,’ which treat of the principles of systematic botany; here, where he has no longer to establish facts, but to arrange ideas, to dispose and summarise, we find Linnaeus thoroughly in his element.
The groundwork of botanical science, he begins, is twofold, classification and naming. The constituting of classes, orders, and genera he calls theoretical classification; the constituting of species and varieties is practical classification. The work of classification carried out by Cesalpino, Morison, Tournefort, and others leads to the establishing of a system; the mere practice of describing species may be carried on by those who know nothing of systematic botany. These expressions of Linnaeus are interesting, because like other remarks of his they show that he placed the establishment and arrangement of the larger groups above the mere distinguishing of individual forms; his disciples to a great extent forgot their master’s teaching, and fancied that the collecting and distinguishing of species was systematic botany. He opposes the system itself, which deals with the relative conceptions of classes, orders, genera, species, and varieties, to a mere synoptical view, serving with its dichotomy only to practical ends. Then comes the often-quoted sentence, ‘We reckon so many species as there were distinct forms created “in principio.”’ In a former place he had said ‘ab initio’ instead of ‘in principio’; instead therefore of a beginning in time he here posits an ideal, theoretical beginning, which is more in accordance with his philosophical views. That new species can arise is, he continues, disproved by continuous generation and propagation, and by daily observation, and by the cotyledons. It is hard to understand how the Linnaean school till far into our own century could have remained firm in a doctrine resting on such arguments as these. Linnaeus’ definition of varieties shows that he understood by the word species fundamentally distinct forms; there are, he says, as many varieties as there are different plants growing from the seed of the same species; and he adds that a variety owes its origin to an accidental cause, such as climate, soil, warmth, the wind; but this is evidently mere arbitrary assumption. Judging by all he says, his view is that species differ in their inner nature, varieties only in outward form. Here, where we find the dogma of the constancy of species for the first time expressed in precise terms,—a dogma generally accepted till the appearance of the theory of descent, we should be justified in demanding proof; but since dogmas as a rule do not admit of proof, Linnaeus simply states his view[32], unless we are to take the sentence, ‘negat generatio continuata, propagatio, observationes quotidianae, cotyledones,’ as proving the assertion that new species never appear. We shall see further on to what surprising conclusions Linnaeus was himself led by his dogma, when he had to take into account the relations of affinity in genera and larger groups. The species and the genus, he continues, are always the work of nature, the variety is often that of cultivation; the class and the order depend both on nature and on art, which must mean that the larger groups of the vegetable kingdom have not the same objective reality as the species and the genus, but rest partly on opinion. That Linnaeus estimated the labours of the systematists after Cesalpino and the contributions of the German fathers of botany up to Bauhin, as they have been judged of in the present work, is shown by paragraph 163, where he explains the word habit, and adds that Kaspar Bauhin and the older writers had excellently divined (divinarunt) the affinities of plants from their habit, and even real systematists had often erred, where the habit pointed out to them the right way. But he says that the natural arrangement, which is the ultimate aim of botany, is founded, as the moderns have discovered, on the fructification, though even this will not determine all the classes. It is interesting therefore to observe how Linnaeus further on (paragraph 168) directs, that in forming genera, though they must rest on the fructification, yet it is needful to attend to the habit also, lest an incorrect genus should be established on some insignificant mark (levi de causâ): but this attention to the habit must be managed with reserve, so as not to disturb the scientific diagnosis.
Linnaeus next lays down with great detail each several rule, which must be observed in establishing species, genera, orders, and classes, and it is here that he displays his unrivalled skill as a systematist. These rules were strictly observed by himself in his numerous descriptive works, and thus a spirit of order and clearness was introduced into the art of describing plants, which gave it at once a different appearance from that which it had received at the hands of his predecessors. Whoever therefore compares the ‘Genera Plantarum,’ the ‘Systema Naturae,’ and other descriptive works of Linnaeus with those of Morison, Ray, Bachmann, or Tournefort, finds so great a revolution effected by them, that he is impressed with the persuasion that botany first became a science in the hands of Linnaeus; all former efforts seem to be so unskilful and without order in comparison with his method. Without doubt the greatest and most lasting service which Linnaeus rendered both to botany and to zoology lies in the certainty and precision which he introduced into the art of describing. But if a reformation was thus effected in botany, as Linnaeus himself took pleasure in saying, it must not be overlooked that the knowledge of the nature of plants was rather hindered than advanced by him. Ray, Bachmann, and in part also Morison and Tournefort, had already liberated themselves to a great extent from the influence of scholasticism, and they still give us the impression of having been genuine investigators of nature; but Linnaeus fell back again into the scholastic modes of thought, and these were so intimately combined with his brilliant performances in systematic botany, that his successors were unable to separate the one from the other.