THE CONSTITUENTS OF THE ASH OF PLANTS 181 



because they are present in the particular soil in which the 

 plant happens to be growing, and have the power of osmosing 

 through the cell-membranes of the root-hairs. Many of 

 them have only been found in a few plants. Among them 

 may be mentioned aluminium, zinc, copper, cobalt, nickel, 

 zirconium, fluorine, and lithium. 



What is frequently spoken of as the selective power of 

 plants is often misunderstood. If a substance is present 

 in a soil, can be made soluble in the hygroscopic water 

 permeating that soil, and can dialyse through the living 

 membrane of the root -hair, absorption ot a certain quantity 

 of it will take place. How much is ultimately absorbed is 

 a question ot the power of the plant to decompose or utilise 

 it after absorption. Many substances which are useless or 

 even deleterious to the plant which takes them up are 

 absorbed continuously until a very large percentage of 

 them is present, because other constituents of the plant 

 decompose them, or because their power of dialysis is such 

 that they are easily removed from the absorbing cells. 

 The possibility of the dialysis by which they are originally 

 taken up is perhaps a question of relationship between the 

 size of their molecules and that of the meshes of the proto- 

 plasmic membranes which bound the cytoplasm on its two 

 faces, abutting on the cell-wall and the vacuolar cavity 

 respectively. Or it may be a question of the protoplasm 

 actually picking them out of the watery stream and passing 

 them into the cell apart from osmosis altogether. This 

 possibility of penetrating into the cell, and the power of 

 subsequently removing the substances therefrom, are the 

 special features of the so-called selective power of the 

 plant, and it is evident that this power is particularly asso- 

 ciated with the disposition of the materials after absorption, 

 more than with the absorption itself. 



We may now turn to the consideration of these varied 

 constituents of the ash, and examine them in detail. The 

 first group, we have seen, is composed of sulphur and 

 phosphorus. Its importance lies in the fact that these 



