CAUSES OF THE CONDITION OF 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 bv 

 evaporation and perceptibly shorter, quickly bend upwards concavely if placed with 

 one side on water; if placed entirely in water they become straight and lono-er. 

 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 tlie cells, and 

 they consequently become smaller; if they are then placed in distilled water they regain their 

 original size (Ueb. d. mechanischen Ursachen der Zellstreckimg, 1877).] 



