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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    distinguished, since their significance is altogether different. In one case the ab- normal shoot of a stock which itself belongs to a variety resembles or reverts to the primitive form; and this therefore is an instance not of the production but of the cessation of a new form. In the botanic garden at Munich there is, for example, a beech-tree with divided leaves, itself a variety, a single branch of which bears the ordinary undivided entire leaves, or has reverted to the primitive form. In the second case new characters not previously displayed arise on particular shoots of a stock. Thus, for instance, single shoots of the myrtle are sometimes found with leaves in alternating whorls of threes, instead of pairs ; but these shoots again produce from the axils of their leaves the ordinary branches with decussate leaves. Knight (see Darwin, l. c. vol. I, p. 375) observed a cherry (the May Duke) with one branch bear- ing fruit of a larger shape which always ripened later. The common ‘moss-rose’ is considered by Darwin (/. c. p.379) to have probably arisen by ‘ bud-variation’ from R. centifolia ; the white and striped moss-roses made their appearance in 1788 from a bud of the common red moss-rose; Rivers states that the seeds of the simple red moss-rose almost always again produce moss-roses1. Those changes which are produced in a plant by the nature of its food and other external conditions must not be confounded with variation. Specimens of the same plant often differ conspicuously in the size and number of their leaves, shoots, flowers, and fruits, according as their supply of food has been abundant or deficient ; deep shade frequently occasions the most striking changes in the habits of plants that usually grow in sunshine ; but these changes are not hereditary; the descendants of such individuals revert, under normal conditions of light and nutrition, to the original characters of the species. Those characters, on the contrary, which may become hereditary or form the groundwork of varieties, arise independently of the direct influence of soil, locality, climate, or other external influences; they appear seemingly without any cause. We must therefore assume either that external impulses which are altogether imper- ceptible first cause an imperceptible deviation in the process of development, which is always extremely complicated, and that this variation gradually increases until it becomes perceptible, or that the processes in the interior of the plant itself react upon one another in such a manner as to cause sooner or later an external change. The fact that wild plants, when cultivated, usually begin to produce hereditary varieties, shows that the change in the external conditions of life disturbs to a certain extent the ordinary process of development; but it does not show that par- ticular external influences produce particular hereditary varieties corresponding to them ; for under the same conditions of cultivation the most different varieties arise simultaneously or successively from the same parent-form. The same is the case also in nature with wild plants; in the same locality under precisely the same vital conditions a number of varieties are often found by the side of their parent-form, and the same variety is often found in the most diverse localities2. It is for this very 1 [See also M. J. Masters, On a pink sport of the Gloire de Dijon rose, Journ. Roy. Hort. Soc. new series, vol. IV, p. 153.—Ed.] 2 Further details on this important subject are given by Nageli in the Sitzungsberichte der kon. bayer. Akad. der Wiss. Dec. 15, 18C3.