CAUSES OF THE CONDITION OF TENSION IN PLANTS. 
79i 
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-Physiologie, 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 0-2 2 2 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 (/. c. p 432). 
Water absorbed by loo parts 
Distension of loo parts by 
by weight of dry wood. 
volume of dry wood. 
Maple 
87 parts 
9.4 
do. 
87 
7-1 
Birch 
97 
7.0 
do. 
91 
8.8 
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. 
