790 MECHANICS OF GROWTH. 



2. ImhihiUon is the term given, as we have already seen, to the capacity' of 

 organised structures to absorb water between their micellae with such force that 

 they are thereby driven apart, their cohesion being partially or entirely overcome, and 

 the whole thus increasing in volume. Loss of water, on the other hand, as by 

 evaporation, causes approximation of the micellae and a corresponding decrease in 

 volume of the whole. Both distension and contraction take place with such force as 

 to overcome external resistances of considerable magnitude. While in closed and 

 thin-walled cells the changes in form and volume are chiefly caused by turgidity, 

 in very thick- walled cells on the contrary with a small cavity (as many bast-fibres and 

 collenchymatous cells) they are brought about mainly by imbibition and desiccation 

 of the cell-wall, and especially when it is to a high degree capable of swelling, in 

 other words is in a state to absorb or give off large quantities of water. In cells 

 with open pores, where there can be no hydrostatic pressure or turgidity, as in 

 wood-cells and vessels with bordered pits, imbibition and the desiccation of the per- 

 forated cell- wall are the only means of changing the size and form of the cell. 



If, as is usually the case with thick cell-walls, the different concentric layers of 

 cellulose have different degrees of capacity for imbibition and swelling (see Book I. 

 Sect. 4), tensions are caused between these layers by the absorption or loss of water, 

 which may even end in the layers becoming detached from one another; as, for 

 example, occurs in transverse sections of thick-walled bast-cells and in starch-grains. 

 But it is not only the quantity of water absorbed and given off that varies in the 

 different layers of a cell-wall, but also the direction in which the water is princi- 

 pally absorbed or allowed to escape between the micellae. Tensions are thus 

 caused which may lead to the production of torsions and oblique fissures, to the 

 rolling or unrolling of spiral bands of the cell-wall, and to a change in the obliquity 

 of the spirals ^. 



All these changes, which are necessarily associated with the tensions of layers 

 that have become convex and concave, take place also in masses of tissue and 

 organs the cells of which have lost their contents and consequently their turgidity, 

 while their cell-walls have become capable of imbibition, or, as it is generally termed, 

 hygroscopic. The layers of cell- walls and the thin-walled masses of tissue which 

 in the living state contain most water, contract most strongly after death and from 

 desiccation; with change of form they become concave, or are ruptured by the 

 contraction of the intermediate lignified tissue. Without entering at present into a 

 detailed consideration of these extremely various phenomena, which, though often 

 of extreme importance in the life of the plant, do not influence growth, it need 

 only be mentioned that on them depend the bursting of most sporangia, anthers, 

 and capsular fruits, the remarkable movements of the awns of various species of 

 Avena and Er odium, as well as those of the Rose of Jericho {Anastatica Mero- 

 chuniica) and of the so-called asthygrometer ^. Of direct importance on the other 

 hand, as respects the mechanical laws of growth, are the changes in volume of the 



^ See Nageli u. Schwendener, Das Mikroskop, p. 427 et seq. (1877). 



^ Compare Cramer, in Nageli u. Cramer's Pflanzen-physiologische Untersuchungen, 1855, Heft 3. 

 p. 28 et seq. ; and Sachs, Experimental-Pliysiologie, p. 429. 



^ Compare Cramer's statements in Wolff's treatise, Die sogenannte Asthygrometer ; ZUrich, 

 1867. 



