The more valuable knowledge which had been gathered up to 1851 was brought together by von Mohl in his often-mentioned work on the vegetable cell with equal perspicuity and conciseness, and current views were critically examined; vegetable physiology generally was expounded at greater length but with less critical sifting in Unger’s text-book of 1855; these were the two books which did most to disseminate a knowledge of the subject up to 1860, and they performed their task with credit; that which appears in Schacht’s books after 1852 under the head of vegetable physiology rests on such imperfect acquaintance with this branch of science, as to diminish rather than increase its reputation.
Passing from this preliminary survey to a more detailed account of the subject, it will be found necessary to keep the history of the sexual theory distinct from other questions in vegetable physiology. This mode of proceeding is required by the fact, that the establishment and further elucidation of the decisive points in the sexual theory were made independently of the rest of physiology, so that the historical continuity would be interrupted and the account rendered obscure by any attempt to connect the development of the theory chronologically with other topics. In like manner the doctrine of the nutrition of plants and of the movement of the sap was developed uninterruptedly and in independence of other physiological matters; it will be advisable therefore to devote a separate chapter to those subjects also. Earlier discoveries respecting the movements of the parts of plants and the mechanics of growth will be briefly recounted in a third chapter.
[CHAPTER I.]
History of the Sexual Theory.
1. From Aristotle to R. J. Camerarius.
It will contribute to a correct appreciation of the discoveries made towards the end of the 17th century by Rudolph Jacob Camerarius and his successors in regard to the sexual relations of plants, if we first make ourselves acquainted with all that was known of the matter up to that time from Aristotle downwards; we shall learn at the same time how extremely unfruitful was the superficial observation of the older philosophy in a question in which inductive research only could lead to real results.
That Aristotle[90] like many others after him reckoned sexual fertilisation among processes of nutrition, and thus failed to perceive the specific and peculiar character of the latter, is shown distinctly by his assertion, that the nutritive and propagative power of the soul is one and the same. This hasty generalisation was associated in Aristotle’s mind with another error arising from very defective experience, which led him to bring sexuality in organisms into causal connection with their movement in space. He tells us in his botanical fragments, that in all animals which have the power of locomotion, the female is distinct from the male, one creature being female, another male, but both being of the same species, as in humankind. In plants on the contrary these powers are combined and the male is not distinct from the female; each plant therefore reproduces itself and emits no fertilising material; and he adds, that in animals which do not move, as those that have shells and those that live attached to some other substance, male and female are not distinguished, for their life resembles that of plants; at the same time they are called male and female by resemblance and analogy, and there is a certain slight distinction. In like manner some trees produce fruits while others do not, though they aid fruit-bearing trees in the production of fruit, as happens in the case of the fig-tree and the caprifig.
In comparison with these views of Aristotle those of his disciple Theophrastus[91] appear to some extent enlightened, and to rest on a wider experience, but even his observation supplies nothing of interest on the subject; for he says that some blossoms of the ‘mali medicae’ produce fruit, and that some do not, and that it should be observed whether the same thing occurs in other plants, which he might easily have done for himself in his own garden. He is more concerned with putting his knowledge into logical order, than with answering the question whether there is any sexual relation in plants. It is certain, he says, that among plants of the same species some produce flowers and some do not; male palms, for instance, bear flowers, the female only fruit[92]; and he concludes the sentence by the remark, that in this lies the difference between these plants, and those which produce no fruit, and that it is obvious that there must be a great difference in the flowers. In his third book ‘De Causis’ (c. 15, 3) he says, that terebinths are some male and some female, and that the former are barren and are therefore called male. That Theophrastus in all these matters trusted to the relations of others is shown by a passage in the same book (c. 18, 1), where he says, ‘What men say, that the fruit of the female date-palm does not perfect itself unless the blossom of the male with its dust is shaken over it, is indeed wonderful, but resembles the caprification of the fig, and it might almost be concluded that the female plant is not by itself sufficient for the perfecting of the fœtus; but this cannot be the case in one genus or two, but either in all or in many.’ We observe the grand style in which the Greek philosopher dismisses this important question, and how far he is from condescending to make an observation for himself.
It appears that in Pliny’s time the hypothesis of a sexual difference in plants had grown up and become confirmed in the minds if not of writers, yet of those who occupied themselves with nature; Pliny in his ‘Historia Mundi,’ describing the relation between the male and female date-palm, calls the pollen-dust the material of fertilisation, and says that naturalists tell us that all trees and even herbs have the two sexes[93].