ABSORPTION OF WATER. 



61 



Godechens analysed four species of Fucus gathered at the mouth of 

 the Clyde, with the following results : 



From the wide differences in the composition of the ash 

 which, as we see, have been found in different plants, we may 

 infer that each kind of plant is endowed with a specific 

 absorbent capacity. It is upon this, in fact, that the " rota- 

 tion of crops " in farming depends. Further, since the quan- 

 tity of a substance absorbed depends, as was stated at the 

 outset, upon its being chemically altered in the metabolism of 

 the plant, we find that the specific absorbent capacity of a 

 plant is an expression of its specific metabolic properties. 

 For example, if we contrast the amount of silica in the ash of 

 a gramineous with that in the ash of a leguminous plant, we 

 cannot but conclude that the former is able to withdraw rela- 

 tively large quantities of absorbed silica from the sphere of 

 osmotic activity and deposit them in its tissues, whereas the 

 latter can only do so to a comparatively very slight extent. 

 But this is a point which would be more appropriately dis- 

 cussed in connexion with the metabolism of the plant, and we 

 will accordingly defer it to a subsequent lecture. 



Godechen's analyses teach further that the absorbent 

 capacities of nearly allied species are very different, and from 

 the following analyses of potatoes made by Herapath it 

 appears that this is true even of varieties of the same species. 



