CAUSES OF THE CONDITION OE TENSION IN PLANTS. 
789 
filtration. It is very different in amount in cells of different kinds, and on it the 
degree of turgidity depends, when the intensity of the endosmotic force of the sap 
and the elasticity of the cell- wall are constant. 
What follows with respect to the turgidity of the individual cell is equally true 
in general of masses of tissue ; only that a much greater variety of phenomena 
may arise in this case according to circumstances. If, for example, a number of 
similar layers of tissue are united into a system, a curvature of the system may 
take place when one layer loses water by evaporation and thus becomes shorter, or 
when it absorbs more water than another layer and thus becomes longer. For 
instance, the primary roots of seedlings which have become partially flaccid by 
evaporation and perceptibly shorter, quickly bend upwards concavely if placed with 
one side on water; if placed entirely in water they become straight and longer. 
Curvatures arise in the same manner when layers of different tissues, united with 
one another, are subjected to variations of turgidity. Stems of the Dandelion for 
instance split lengthwise and placed in water roll up in a spiral manner, the outside 
being concave, because the medullary parenchyma absorbs much more water, and 
consequently, from the extensibility of its cell-walls, expands more than the epidermis 
or the cortex, which absorb water more slowly, and whose cell-walls are besides not 
so extensible ^ 
As a single cell, with increasing turgidity, opposes greater resistance to forces 
which tend to change its form, so also a mass of tissue becomes more rigid when all 
its cells are more strongly turgid, and vice versa. If, for example, a cylinder of pith is 
cut out from a growing internode, it is flaccid and flexible ; but if it is placed for a 
quarter or half an hour in water, it not only becomes considerably longer, but also 
very rigid and even brittle in consequence of all its cells becoming rapidly filled with 
water. This effect is still more visible when the pith is surrounded by other less 
extensible tissues, as in an uninjured internode. If this internode has become 
flaccid from transpiration, and it is placed in water, the pith very soon begins to 
become turgid and to expand ; but since it is surrounded by other tissues of 
different properties, it must stretch them in order to lengthen itself ; this is only pos- 
sible however until the elasticity of these layers is in equilibrium with the tendency of 
the pith to expand. In this case the elongation of the whole caused by the turgidity 
of the pith is much less than that of the pith alone would be ; but on the other hand 
there is now a violent tension between the pith and the surrounding tissues, in 
consequence of which the whole internode appears very rigid or but slightly flexible. 
The whole internode may be compared to a cell the contents of which are 
represented by the pith, its cell-wall by the surrounding tissues. If the pith loses 
water the whole becomes smaller, the passively stretched tissues contracting elas- 
tically; and since the tension is thus decreased, the whole becomes more flaccid ; 
the reverse when the change is in the opposite direction. 
micellar interstices is clear from the fact that the amount of soluble substances contained in the 
water is altered by the filtration. 
* [De Vries has shown that the turgidity of cells may be diminished by placing them in 
solutions of neutral salts (KNO3, Na CI) of 4-6 per cent. ; water is withdrawn from the cells, and 
they consequently become smaller; if they are then placed in distilled water they regain their 
original size (Ueb. d. mechanischen Ursachen der Zellstreckung, 1877).] 
