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
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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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    more in detail, that primary roots with strong positive geotropism, as well as secondary rootlets, when growing in moderately damp air, deviate from their vertical or oblique direction when there is a moist surface near them. Under these circumstances a curvature concave to the moist surface takes place at the region below the apex where there would otherwise be a downward curvature, the apex being by this means con- ducted towards the moist surface so that it may penetrate into the moister soil or grow in contact with it. The apparatus represented in Fig. 453 is well adapted to exhibit this phenomenon. It consists of a zinc frame a a covered below with wide-meshed network, thus forming a sieve hanging obliquely and filled with moist sawdust ff. The seeds g gg germinate in the sawdust, their roots penetrating at first vertically downwards into it. When the apex of a root escapes through the network into air which is not too dry, it turns towards the moister surface h-m, its geotropism being thus evidently overcome1. Sect. 22. Unequal Growth2. Our observations have hitherto had reference ' almost exclusively to the growth of multilateral or polysymmetrical organs, such as erect stems and descending roots. Organs of this kind offer the simplest example of growth taking place equally on all sides. But they form only a small minority, tt since not only a large number of primary stems like those of Hepatic®, Rhizocarpe®, and Selaginelle®, but also by far the greater number of erect stems, and all leaves, display a decidedly bilateral structure, i. e. two sides of their axis of growth exhibit different characters. With this bilateral organisation is also usually connected a difference in the growth of the two unequal sides, which causes curvatures and hence changes in the position of the apex. The two unequal sides of bilateral organs must also be acted on differently by external agencies which affect growth, such as light, gravitation, and pressure. We do not attempt here to solve the question of the causes which produce the bilateral structure in any particular case; i it need only be shown incidentally that this structure of lateral organs (as we have already seen in Book I, Sect. 27) is probably always brought about by internal causes, and is independent of the action of external circumstances. This is in general at once evident from the fact that the median plane of bilateral appendi- calar organs has always a perfectly definite geometrical relation to the axial structure which bears them, and that moreover in the dark and under the influence of slow rotation round a horizontal axis, which eliminates the effect of gravitation, the bilateral structure and relation to the axis remain unchanged. But before we proceed to the consideration of the growth of bilateral organs, it must be premised that even in multilateral erect stems and vertically descending roots growth does not always proceed equally and with equal rapidity on all sides of the longitudinal axis; it is much more common for first one side and then another of the organ to grow more rapidly than the rest, curvatures being thus caused the con- vexity of which always indicates the side that is at the time growing most rapidly. 1 [For a further detailed series of experiments on the influence of gravitation on growth, see Sachs, Flora, 1873, No. 21, and Arb. des bot. Inst. Wurzburg, 1873, Heft 3.—Ed.] 2 A. B. Frank, Die natiirliche wagerechte Richtung von Pfianzentheilen (Leipzig, 1870). The views propounded in Frank’s treatise are opposed by H. de Vries in the second Heft of the Pro- ceedings of the Wiirzburg Bot. Inst. 1871, p. 223 et seq—See also Hofmeister, Allgemeine Morpho- logic der Gewfichse, Leipzig, 1868, Sect. 23, 24.