

CAUSES OF THE CONDITION OF TENSION IN PLANTS, 791 



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, viz. 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. 



3. Growth itself must cause states of tension in the layers of a cell-wall or of 

 the tissue of which an organ is composed, if the layers, although firmly united to 

 one another, grow unequally. It is however much more difficult to understand 

 the modifications of tension due to growth than those due to turgidity and imbi- 

 bition, as the former cannot be altered artificially without a material change being 

 caused also in the latter. Since the growth of every organised structure, such as 

 a cell-wall, can only proceed so long as it is permeated with water, and since 

 moreover the growth of the entire cell requires it to be in a turgid condition, and 

 this condition itself has an influence on growth, it is extremely difficult to decide 

 how far each of these phenomena is the cause of the other. If by growth we under- 

 stand, according to the definition already given, only permanent and irreversible 

 changes of organisation, affecting in the first place the micellar structure of the 

 organism, it may be assumed, in accordance with the present state of our know- 

 ledge, that growth is always preceded by imbibition and turgidity, and that it is the 



^ The measurements of Laves given below illustrate these relative changes of dimension. (See 

 Sachs, Experimental-Physiologic, p. 431.) 



In the direction of In the direction of In the direction of the 



the axis. the radius. circumference. 



Maple 0.072 3-35 6.59 



Birch 0-222 3*86 6.59 



Oak 0.400 3.90 7.55 



Fir 0-076 2.41 6-18 



The change in volume of wood was investigated by Weisbach {I. c, p 432). 



Water absorbed by loo parts Distension of lOo parts by j 



by weight of dry wood. volume of dry wood. 



Maple 87 parts 9-4 



do. 87 7-1 



Birch 97 7'0 



do. 91 S'S 



Oak 60 ■ 7-2 



do. 91 7«8 



Fir 94 5.7 



do. 130 5-1 



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. 



