CAUSES OF THE CONDITION OF TENSION IN PLANTS. 709 



turgid. The molecular pores through which the water set in motion by endosmose 

 forces its way into the cells are essentially different from orifices ; the former are so 

 small that their diameter is completely under the control of the molecular forces, 

 while even the smallest orifice withdraws at least the middle portion of its space 

 from the influence of the molecular action of the substance that bounds it. Micro- 

 scopic openings, like the pores of bordered pits, are orifices of this latter kind, and 

 are excessively large compared with the molecular pores through which endosmose 

 acts. Cells with pits penetrating the cell-wall cannot therefore be turgid, because 

 any tension however small between cell-wall and contents is at once neutralised by 

 the superfluous sap becoming pressed out through the orifices. It is indeed possible 

 for water to be forced out in this way even through closed cell-walls, but only when 

 the turgidity is very great, and the hydrostatic pressure of the cell-sap on the per- 

 fectly tense cell-wall is sufficient to force out the water through the molecular pores ^ 

 The resistance offered by the cell-wall to this may be called resistance to 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 are united 

 with one another and subjected to a variable amount 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, ex- 

 pands more than the epidermis or the cortex, which absorb water more slowly, 

 and whose cell-walls are besides not so extensible. 



As the cell, with increasing turgidity, opposes greater resistance to forces which 

 tend to change its form, 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 ah its cells becom.ing rapidly filled wdth 

 water. This effect is still more visible when the pith is surrounded by other less 

 elastic tissues, as in an uninjured internode. If this internode withers, /. e. becomes 

 flaccid from transpiration, and is placed in water, the pith very soon begins to 



^ That the water which filters through under such circumstances actually passes through 

 molecular pores is clear from the fact that the amount of soluble substances contained in the 

 water is altered by the filtration. 



