The Sexes of Flowering Plants.
As soon as men began to raise plants in gardens, or even earlier, they must have remarked that plants produce seeds, and that seeds develop into new plants. The Greeks (Empedocles, Aristotle, Theophrastus) recognised that the seed of the plant answers to the egg of the animal, which is substantially though not literally true. None of the three understood that a process of fertilisation always, or almost always, precedes the production of seed. Had the date-palm, whose sexes are separate, and which has been artificially fertilised from time immemorial, been capable of cultivation in Greece, Aristotle would not have said that plants have no sexes, and do not require to be fertilised. His pupil, Theophrastus, knew only by hearsay of the male and female date-palms, and affirmed that both bear fruit. Pliny, three hundred years later, called pollen the fertilising substance, and gave it as the opinion of the most competent observers that all plants are of two sexes. The revivers of botany paid no attention to pollen or the function of the flower; it is more surprising that in the following century Malpighi, who had diligently studied the development of the plant-embryo, should give so superficial an account of the stamen and its pollen. About the same time Grew and Millington expressed their conviction that "the attire" (anthers) "doth serve as the male, for the generation of the seed."[9] A few years later Ray[10] speaks of the masculine or prolific seed contained in the stamens. In 1691-4 Camerarius, professor at Tübingen, brought forward clear experimental proof that female flowers, furnished only with pistils, produce seeds freely in the neighbourhood of the male or staminate flowers, but fail to do so when isolated. He distinctly inferred that the anthers are male organs and the pistil the female organ. The claim set up on behalf of Linnæus that he demonstrated, or helped to demonstrate, the sexes of flowering plants has little foundation in fact. To make out such details of the process of fertilisation as the formation of pollen-tubes, the penetration of the ovules and the fusion of nuclei required the improved microscopes of the nineteenth century.
The almost universal presence both in plants and animals of a process of fertilisation is a fact whose physiological meaning we but imperfectly grasp. Modern research has shown that the pollen-tube is exceptional and confined to the flowering plants; the motile filament of cryptogams, analogous to the spermatozoon of animals, is no doubt a relatively primitive structure, which gives one of the strongest indications of the common origin of all forms of life.
[6] Ray came at last to believe that fossils were the remains of actual organisms, but he was still much hampered by his theological views.
[7] The second of the two has actually been so treated, but without mention of Perrault's name.
[8] See Krause's Life of Erasmus Darwin.
[9] Grew's Anatomy of Plants, 1682.
[10] Wisdom of God, 1691.
PERIOD III
1741-1789