VEGETABLE ABSORPTION. 23 



greater part of the rain which falls upon the tree, is made to 

 drop from the leaves at the exact distance from. the trunk, 

 where, after it has soaked through the earth, it will be re- 

 ceived by the extremities of the roots, and readily sucked 

 in by the spongioles. We have here a striking instance of 

 that beautiful correspondence, which has been established 

 between processes belonging to different departments of na- 

 ture, and which are made to concur in the production of re- 

 mote effects, that could never have been accomplished with- 

 out these preconcerted and harmonious adjustments. 



The spongioles, or absorbing extremities of the roots, are 

 constructed of ordinary cellular or spongy tissue: and 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 

 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 tlie stem of the plant. 

 The most probable explanation of this phenomenon is, that 

 the progressive movement of the fluid is produced by alter- 

 nate contractions and dilatations of the cells themselves, 

 which compose the texture of the plant: these actions being 

 themselves referrible 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 vis- 

 cid or glutinous a consistence 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 exhi- 

 bited when the saline solutions of a certain strength arc pre- 

 sented to the roots: the water of the solution, with only a 



