Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer.
- Date:
- 1875
Licence: Public Domain Mark
Credit: Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
838/880 page 822
![them; and they are usually very variable. Wichura showed, from his own observ- ations and those of Gartner, that hybrid pollen produces a greater variety of forms in its progeny than does the pollen of true species. The results of hybridisation are important with respect to the theory of sexuality, because there is no boundary-line or essential distinction between the self-fertilisation of pure species or varieties and their fertilisation by other species or varieties; and be- cause in the latter case—in other words in hybridisation—certain peculiarities of sexual differentiation and union are rendered more evident. The two extremes of the con- ditions under which a fertile union of sexual cells is possible lie at a great distance from one another, but are connected by very numerous transitions. One extreme is presented in the genus Rhynchonema and in some Saprolegnieae, where a fertile sexual union of sister-cells takes place regularly; the other extreme is furnished in genus-hybrids, where the uniting cells belong to very different forms of plants whose descent from a common ancestor dates back to a remote antiquity. But the great majority of phenomena in the vegetable kingdom show that sexual union is usually most productive when the cells stand neither in too close nor in too remote an affinity to one another ; self-fertilisation is in the vast majority of cases as carefully avoided as the hybridisation of different species or genera. The phenomena may be comprised in the statement that the original form of sexual differentiation was probably the simultaneous formation of male and female organs in close juxtaposition on the same plant, but that sexual union is more potent and more favourable for the maintenance of the race when the closely contiguous cells do not unite, but those of different descent, a certain mean amount of difference of descent being established as the most favourable. This mean of the difference of descent associated with a maximum of sexual potency is obtained when the sexual cells belong to different individuals of the same species1. The phenomena of structure described in the preceding paragraphs which are manifested in polygamy, diclinism, dichogamy, dimorphism, the impotence of pollen on the stigma of the same flower (as in Corydalis and Oncidium), the mechanical contrivances for rendering self-fertilisation impossible (as in Aristolochia Clematitis, many Orchideae, &c.), are different means for promoting the cross-fertilisation of individuals belonging to the same species or for rendering it alone possible. CHAPTER VII. THE ORIGIN OF SPECIES. Sect. 33.—Origin of Varieties. The characters of plants are transmitted to their descendants, or, in other words, are hereditary. But, in addition to the inherited properties, new characters may arise in a smaller or larger number of the descend- ants of a plant which were not possessed by the parent-plants. Thus, for example, Descemet obtained in 18032, among the seedlings from Robinia Pseud-acacia, an 1 [See Darwin, Variation of Animals and Plants under Domestication, vol. II, chap, xvii, where several illustrations of the law are given.—Ed.] 5 See Chevreul, Ann. des sci. nat. 1846, vol. VI, p. 157. [Journ. Roy. Hort. Soc. vol. VI, 1851. p. 61.]](https://iiif.wellcomecollection.org/image/b21981437_0838.jp2/full/800%2C/0/default.jpg)
