PASSAGE OF WATER FROM PLANT CELL. I I I 



We may now turn to the other theory, here designated for con- 

 venience the " attraction " theory. 



In ordinary plant tissue containing intercellular spaces the fol- 

 lowing conditions are present. In the interior of the cell first 

 we find a cell sap of varying concentration, the quantity of which 

 is conditioned principally by the size of the cell. Outside of this 

 is the protoplasmic layer containing a considerable quantity of 

 water, usually from 60 to 90 per cent. This is in direct contact 

 with the cell wall, which in turn contains from 30 to 60 per cent, 

 of water. Bordering on the wall is the free atmosphere of the 

 intercellular space. 



The water contained in the wall and in the protoplasm is water 

 of imbibition, in the limited sense of the word. The term " cap- 

 illary water " is usually applied to water of imbibition in spaces 

 large enough to be readily detected, and in which most of the 

 molecules of the liquid are out of range of the molecular forces of 

 the substance with which the liquid is in contact. These extreme 

 conditions are connected by insensible gradations dependent upon 

 the size of the minute chambers in which the water is contained. 

 The forces acting to inject the liquid into the substance are pri- 

 marily the same in either case, namely, the great attraction of the 

 molecules of the one for those of the other. 



The fact that a substance swells implies that the molecules of 

 the liquid have a greater attraction for those of the substance than 

 the latter have for each other, at least than they have for each 

 other in some directions. The result is that the particles of 

 liquid prv and wedge themselves between those of the substance, 

 forcing the latter apart. The particles later to enter, however, 

 cannot come as near the substance as did the first ones, and hence 

 their action is less intense. Perhaps the physicist more accurately 

 describes this wedging power of the liquid when he says that it 

 depends upon the concavity of the meniscus at the end of the 

 minute water columns entering the substance. The smaller the 

 capillarv canals the more concave is the meniscus, and hence the 

 greater will be the penetrating force. Therefore, as the substance 

 becomes more saturated the force rapidly decreases. A substance 

 that cannot swell is simply one in which the molecules have a 



