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.
749/880 (page 733)
![But in contrast to the phenomena which have now been described, external pressure also sometimes causes growth at places where otherwise there would be none. Thus Pfeffer has shown1 that certain hyaline superficial cells on both of the flat sides of the gemmae of Marchantia possess the power of growing out into tubular root-hairs when they remain in contact for some time with a moist solid body; while contact with water produces no effect of the kind. These cells usually develope into root-hairs only when their outer surface is directed downwards, while those on the upper side, not being in contact with a solid body, do not grow out. This, as we shall see presently, is an effect of gravitation, which is however over- come by the action of the slight continuous contact, since this causes the cells on the upper side of the gemmae also to grow out into root-hairs. The ‘ haustoria’ of Cuscuta and Cassytha and the adhesive discs on the tendrils of the Virginian Creeper are only formed, as was shown by v. Mohl, on the continuous contact of the surfaces of the tissue with a solid body; and this has been confirmed by recent ex- periments of Pfefifer’s (l.c. p. g6)2. In these cases a growth combined with cell- | division and differentiation of tissue is caused by contact or slight pressure on a ' part of the organ, and would not take place without this pressure. These haustoria and adhesive discs thus formed are altogether indispensable for the life of the plant; for Cuscuta is nourished exclusively by the haustoria which penetrate into the tissue of the host; and it is by the formation of adhesive discs on the tendrils that the Virginian Creeper is enabled to climb up walls. If the tendrils do not meet with any solid body to which they can attach themselves by means of these discs, they dry up and fall off, while those which have formed discs increase in thickness ! and become woody. The injurious effect on growth of an external pressure on the cells is very evident in the formation of the annual rings in wood. In the earlier editions of this work I called attention to the fact that the larger radial diameter of the w'ood-cells in the portion of the rings formed in the spring, and their smaller radial diameter in the por- tion formed in the autumn, may possibly depend on a difference in the pressure from the surrounding bark to which the cambium and the wood are subject, this pressure being less, as we have shown, in the spring, and constantly increasing during the sum- mer. This hypothesis has been fully confirmed by H. de Vries’s recent investigations3. In branches two or three years old he increased the pressure of the bark in the spring by firmly winding strings round them at particular places. ‘ The experiment show'ed in all cases, firstly, that the absolute thickness of the annual ring was less beneath the liga- ture than the mean thickness of the same annual ring at some distance above or below that spot. In several instances the difference was so considerable that the spot where the experiment was made appeared of considerably less diameter even to the naked eye, and this effect was increased by the formation of cushions of w'ood immediately above and below the ligature. Secondly, the absolute thickness of the ‘autumnal layer’ of wood (up to the middle of August, w'hen the increase in diameter of the tree on which the observations were made ceased), was always greater, and generally considerably so, than the normal thickness at the spot where the experiment was made. In the trees 1 Arbeiten des Bot. Inst, in Wurzburg, Heft I, p. 22. 1 [See also Darwin, On the Movements and Habits of Climbing et seg.—Ed.] ’ H. de Vries, Flora 1872. No. 16. Tlants, London 1865, p 84](https://iiif.wellcomecollection.org/image/b21981437_0749.jp2/full/800%2C/0/default.jpg)
