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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    reason—because varieties are to so great an extent independent of external influences —that they are hereditary. A change produced in a plant by moisture, shade, or any similar cause, is for the same reason not hereditary, because its descendants, when placed under other vital conditions, acquire again other non-permanent characters. That hereditary characters, or those which may become so, are not produced by ex- ternal influences, is proved most conclusively by the fact that seeds from the same fruit produce different varieties, either entirely so or together with the inherited parent-form. Although the production of varieties and the form they assume are not the direct results of external influences, yet the continuance of the existence of a variety may be determined by these influences. When a variety has arisen, the question arises whether it will thrive best in damp or in dry ground, in sunny or shady places, and so forth; whether it can reproduce itself under these circumstances, or whether it will perish. The conclusion follows that hereditary varieties arise independently of direct external influences, but that the continuance of their existence depends on external causes. A variety which occurs only in a particular locality is not produced by the conditions of this particular locality; but it alone furnishes the peculiar vital conditions which this particular variety requires, while other varieties which have arisen at the same place disappear. It has already been shown in Sect. 32 that hybrids show in general a tendency to the production of varieties. Two different sets of hereditary characters are com- bined in a hybrid, and there is hence a strong tendency towards the formation of new characters which may be more or less hereditary. Hybridisation is therefore one of the most important means at the command of the horticulturist for disturbing the constancy of inherited characters and producing a number of varieties from two dis- tinct ancestral forms1. But even the ordinary sexual union of two individuals of a species, as in dioecious, dichogamous, or dimorphic plants, may be considered as a kind of hybridisation; in these cases also the individuals which unite must cer- tainly be different, since otherwise their cross-fertilisation would be no more pro- ductive than self-fertilisation. In these cases therefore two sets of characters which differ, though it may be but slightly, also unite in the descendants; and if a hybrid from two different species exhibits a strong tendency to variation, the cross-fertil- isation of two different individuals of one and the same species may at least give rise to a slight tendency in the same direction. It is therefore probable that in the cross-fertilisation of different individuals—towards which there is always a tendency in nature even in hermaphrodite flowers—we have a perpetual cause of variation in plants. But this is by no means the only cause of variation, as is shown by the existence of bud-variation, and by the reflection that there must always be a slight difference between individuals which produce a variable progeny. A great number of facts point to the conclusion that almost every plant has a tend- ency to vary continually and in different directions, while every new character which is not produced directly by external agencies tends at the same time to become hereditary. 1 See also Naudin, Compt. rend. 1864, vol. LIX, p. 837. [Journ. Roy. Hort. Soc. new series, vol. I, p. 1.]