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
Catalogue details

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.

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    individual without spines; Duchesne, in 17611, among seedlings of the strawberry, one with simple instead of trifoliolate leaves ; and Godron2, among seedlings of Datura Talula, one with smooth instead of spiny capsules. The characters which arise in single descendants are often only individual, i. e. they are not again transmitted to their descendants. Thus the seeds of the un- armed Robinia produced again spiny plants resembling, not their immediate ancestor, but more remote ones ; while in other cases the new character is hereditary, though at first perhaps only partially so, the new form making its appearance only in a certain proportion of the descendants, while the others revert to the original form, as in Duchesne’s unifoliolate strawberry. When a new character is transmitted by inheritance to new generations, the number of individuals that revert to the primitive form often decreases from gener- ation to generation, or the hereditary permanence of the new character increases ; they become more and more constant, and sometimes even as much so as those of the primitive form. Such new constant forms are termed Varieties3. The same parent-form may produce a smaller or larger number either simul- taneously or in succession, sometimes even hundreds of new forms; and this is especially the case with cultivated plants. The enormous number of varieties of the dahlia, differing in the colour, size, and form of the flowers and in their mode of growth, now cultivated in our gardens, have been derived since 1802 from the simple yellow-blossomed primitive form of Dahlia variabilis. The great variety of pansies, distinguished chiefly by the colour of their flowers, have resulted since 1687 from the cultivation of the Viola tricolor of our fields with small flowers almost uniform in colour4. Still more numerous are the varieties of Cucurbita Pepo, differing not only in the form of their fruit but also in all other characters ; and the same is the case with the cabbage (Brassica oleracea) and a vast number of other cultivated plants. Some plants have a special tendency to variation ; among native species, for example, the fruticose Rubi, and those of Rosa and Hieracium; others, on the con- trary, are distinguished by great constancy in their characters, as for example rye, which has as yet produced no hereditary varieties, notwithstanding long cultivation; while the nearly related species of wheat (especially Triticum vulgare, amyleum and Spella) are distinguished by a number of old varieties and an ever-increasing number of new ones. By far the greater number of hereditary varieties are the result of sexual repro- duction ; among Phanerogams the new characters appear suddenly in individual seedlings, which differ at once from the parent-plant in these respects. Sometimes however it happens that particular buds develope differently from the other shoots of the same stock; and of this Bud-variatioiv' two different cases must be carefully 1 For further details, see Usteri, Annalen der Botanik. vol. V, p. 40. 2 See Naudin, Compt. rend. 1867, vol. LX1V, p. 929. 3 For examples, see Hofmeister, Allgemeine Morphologie, p. 565. 4 Darwin, The Variation of Animals and Plants under Domestication, vol. I, p. 368 et seq. s [T. Meehan adduces a number of remarkable instances of bud-variation in which hybrid- isation could not have taken any part;—in Rubus which rarely produces seeds in the wild state, Convolvulus Batatas, which seldom flowers in America, &c. See Proceedings of the Philadelphia Acad, of Nat. Sci. Nov. 29, 1870.—Ed.]