VEGETABLE ABSORPTION. 19 



sufficiently account for the simple entrance of the 

 f^uid, they are inadequate to explain its continued 

 ascent through the substance of the root, or along 

 the stem of the plant. The most probable expla- 

 nation of this phenomenon is that the progressive 

 movement of the llnid is produced by alternate 

 contractions and dilatations of the cells themselves, 

 which compose the texture of the plant ; these 

 actions being themselves referable 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 consistence to pass 

 readily through them are liable to obstruct or en- 

 tirely block up these passages. Thus if the spon- 

 gioles 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 solu- 

 tion, with only a small proportion of the salts, 

 being taken up ; and the remaining part of the 

 fluid being found to be more strongly impregnated 

 with the salts than before this absorption had taken 

 place. It would appear, however, that all this is 

 merely the result of a mechanical operation, and 

 that it furnishes no evidence of any discriminating 

 faculty in the spongiole ; for it is found that, pro- 



