22 THE VITAL FUNCTIONS. 



a hygroscopic power. But though these principles 

 may sufficiently account for the simple entrance 

 of the fluids, they are inadequate to explain its 

 continued ascent through the substance of the 

 root, or along the stem of the plant. The most 

 probable explanation of this phenomenon is that 

 the progressive movement of the fluid is produced 

 by alternate contractions and dilatations of the 

 cells themselves, which compose the texture of 

 the plant ; these actions being themselves refer- 

 able to the vitality of the organs. 



The absorbent power of the spongioles is 

 limited by the diameter of their pores, so that 

 fluids which are of too viscid or glutinous a con- 

 sistence to pass readily through them are liable 

 to obstruct or entirely block up these passages. 

 Thus if the spongioles be surrounded by a thick 

 solution of gum, or even of sugar, its pores will 

 be clogged up, scarcely any portion of the fluid 

 will be absorbed, and the plant will wither and 

 perish ; but if the same liquids be more largely 

 diluted, the watery portion will find its way 

 through the spongioles, and become available 

 for the sustenance of the plant, while the greater 

 part of the thicker material will be left behind. 

 The same apparent power of selection is exhibited 

 when saline solutions of a certain strength are 

 presented to the roots ; the water of the solution, 

 with only a small proportion of the salts, being 

 taken up ; and the remaining part of the fluid 



