When the difference is great, as we see in catkin-bearing plants, this depends largely on many of the species in this, as well as in the next or dioecious class, being fertilised by the aid of the wind; for the male flowers have in this case to produce a surprising amount of incoherent pollen. (Introduction/16. Delpino 'Studi sopra uno Lignaggio Anemofilo' Firenze 1871.) Some few monoecious plants consist of two bodies of individuals, with their flowers differing in function, though not in structure; for certain individuals mature their pollen before the female flowers on the same plant are ready for fertilisation, and are called proterandrous; whilst conversely other individuals, called proterogynous, have their stigmas mature before their pollen is ready. The purpose of this curious functional difference obviously is to favour the cross-fertilisation of distinct plants. A case of this kind was first observed by Delpino in the Walnut (Juglans regia), and has since been observed with the common Nut (Corylus avellana). I may add that according to H. Muller the individuals of some few hermaphrodite plants differ in a like manner; some being proterandrous and others proterogynous. (Introduction/17. Delpino 'Ult. Osservazioni sulla Dicogamia' part 2 fasc 2 page 337. Mr. Wetterhan and H. Muller on Corylus 'Nature' volume 11 page 507 and 1875 page 26. On proterandrous and proterogynous hermaphrodite individuals of the same species, see H. Muller 'Die Befruchtung' etc. pages 285, 339.) On cultivated trees of the Walnut and Mulberry, the male flowers have been observed to abort on certain individuals, which have thus been converted into females; but whether there are any species in a state of nature which co-exist as monoecious and female individuals, I do not know. (Introduction/18. 'Gardener's Chronicle' 1847 pages 541, 558.)

The third Class consists of dioecious species, and the remarks made under the last class with respect to the amount of difference between the male and female flowers are here applicable. It is at present an inexplicable fact that with some dioecious plants, of which the Restiaceae of Australia and the Cape of Good Hope offer the most striking instance, the differentiation of the sexes has affected the whole plant to such an extent (as I hear from Mr. Thiselton Dyer) that Mr. Bentham and Professor Oliver have often found it impossible to match the male and female specimens of the same species. In my seventh chapter some observations will be given on the gradual conversion of heterostyled and of ordinary hermaphrodite plants into dioecious or sub-dioecious species.

The fourth and last Class consists of the plants which were called polygamous by Linnaeus; but it appears to me that it would be convenient to confine this term to the species which coexist as hermaphrodites, males and females; and to give new names to several other combinations of the sexes--a plan which I shall here follow. Polygamous plants, in this confined sense of the term, may be divided into two sub-groups, according as the three sexual forms are found on the same individual or on distinct individuals. Of this latter or trioicous sub-group, the common Ash (Fraxinus excelsior) offers a good instance: thus, I examined during the spring and autumn fifteen trees growing in the same field; and of these, eight produced male flowers alone, and in the autumn not a single seed; four produced only female flowers, which set an abundance of seeds; three were hermaphrodites, which had a different aspect from the other trees whilst in flower, and two of them produced nearly as many seeds as the female trees, whilst the third produced none, so that it was in function a male. The separation of the sexes, however, is not complete in the Ash; for the female flowers include stamens, which drop off at an early period, and their anthers, which never open or dehisce, generally contain pulpy matter instead of pollen. On some female trees, however, I found a few anthers containing pollen grains apparently sound.

Charles Darwin

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