790 
MECHANICS OF GROWTH. 
2. Imbibition 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 {Anasiatica hiero- 
chunticd) 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 Nägeli u. Cramer's Pflanzen-physiologische Untersuchungen, 1855, Heft 3. 
p. 2^ et seq. ; and Sachs, Experimental-Physiologic, p. 429. 
^ Compare Cramer's statements in Wolff's treatise, Die sogenannte Asthygrometer ; Zürich, 
1867. 
