PHYSIOLOGY. 279 



they imbibe the fluids which are in contact with them, partly 

 by capillary action, and partly, also, by what has been termed 

 a hygroscopic power. But though these principles may suffi- 

 ciently account for the simple entrance of the fluid, 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 tnat 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 them- 

 selves 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 though them, are 

 liable to obstruct or entirely block up these passages. Thus 

 if the spongiole 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 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 me- 

 chanical operation, and that it furnishes no evidence of any 

 discriminating faculty in the spongiole ; for it is found that, 

 provided the material presented be in a perfect state of solu- 

 tion and limpidity, it is sucked in with equal avidity, whether 

 its qualities be deleterious or salubrious. 



Solutions of sulphate of copper, which is a deadly poison, 

 are absorbed in large quantities by the roots of plants, which 

 are immersed in them ; and water which drains from a bed 

 of manure, and is consequently loaded with carbonaceous 

 particles, proves exceedingly injurious when admitted into the 

 system of the plant, from the excess of. nutriment it contains, 

 But in the ordinary course of vegetation, no danger can arise 

 from this general power of absorption, since the fluids which 



