12 OF VITAL MOTION. 



intercellular passages as well as the cavities of the 

 cells, and to pass without difficulty through organic 

 membranes, except these be thickened and altered by- 

 solid deposits. During the winter, however, or, at 

 any rate, in times of severe frost, (as we learn from 

 M. Biot,) the sap of the same plant flows chiefly from 

 the branches towards the roots, so that at these times 

 the former organs would seem to exercise the office 

 which generally belongs to the latter. In plants, also, 

 where the roots are sparingly developed, and infixed in 

 arid and moving sand, or in parasites, whose dry, 

 aerial roots are every-way unfitted to the function of 

 absorption, it is clear that the fluids necessary to the 

 wants of the economy must find entrance by other 

 channels. In the lower cellular plants, also, which 

 are adherent to dry and barren rocks, the surface 

 which exercises the function of absorption is that 

 which corresponds to the aerial system of more perfect 

 plants. It would appear, therefore, that the stem and 

 root, with their several organs respectively, may be 

 the channel of absorption, a varying degree of import- 

 ance being assigned to either in different plants ; and 

 this we may understand to be possible, from the law 

 of analogy which has been already set forth in another 

 work. It would appear, also, that there are periodical 

 changes in the same plant, so that no one plan can be 

 considered as constant a supposition which is the 

 more probable from the fact, that the sap infiltrates 



