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 tound in a few plants. Among them 
may be mentioned aluminium, zinc, copper, cobalt, nickel, 
zirconium, fluorine, and lithiwm. 
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, abuttmg on the cell-wall and the vacuolar cavity 
respectively. Orit 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, ig composed of sulphur and 
phosphorus. Its importance lies in the fact that these 
