CAUSES OF THE CONDITION OF TENSION IN PLANTS. 71 L 



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 variable 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 Erodium, as well as those of the Rose of Jericho {Anastafica 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 

 wood and bark of trees which accompany the variation in the quantity of water 

 they contain, and the very powerful tension between them thus caused in woody 

 plants, to which I shall again recur in detail. The attention of the student need 

 now only be called to one point, vh. that when wood distends on imbibition or 

 contracts on desiccation, this is caused entirely by the alteration in form and 

 volume of the cell-walls, since turgidity cannot take place in wood as it does in a 

 tissue consisting of closed cells. The distension and contraction of wood when it 

 absorbs or loses water are very different in different directions, strongest in the 

 tangential, weaker in the radial, weakest of all in the longitudinal direction^. This 

 is the cause, for instance, of the longitudinal splits in woody stems when they become 

 dry, which close again when water is absorbed ; and the changes of dimension due 

 to these phenomena take place with extraordinary force. 



* Compare Cramer's statements in Wolff's treatise, Die sogenannte Asthygrometer ; Zurich, 

 1867. 



- This is the case, according to Laves, in the changes of dimension. (See Sachs, Experimental- 

 Physiologie, p. 431.) 



In comparing the change in volume with the amount of water absorbed, it must be borne in mind 

 that the numbers in which the latter is expressed do not give merely the amount of water imbibed 

 by the cell-walls, which alone causes the distension, but also that retained in the cavities by capillary 

 attraction. It may therefore happen that there appears a smaller increase in volume when a larger 

 quantity of water is absorbed. 



