PLANT NUTRITION. 15 
relative proportions in the plant, while the quantity in 
any given sample of the soil from which it must be de- 
rived, is sometimes so small as to elude detection. The 
plant in this case, or some part of it, is so greedy, if we 
may so say, for this particular substance, that it absorbs 
all within its reach, and stores it up in its tissues or uses 
it some way, the demand ensuring supply. On the 
-other hand, the soil may contain a large quantity of 
some particular ingredient which is incapable of being 
absorbed, or which the plant does not or cannot make 
use of, and, in consequence, none is found within the 
plant. The supply is present, but there is no demand. 
The different physical requirements of the plant sup- 
ply also the explanation of the fact that different plants, 
grown in.the same soil, supplied with the same food, yet 
vary so greatly in chemical composition. Thus, when 
wheat and clover are grown together, and afterwards 
analyzed, it is found that while lime-is abundant in the 
clover, it is relatively in small quantity in the wheat ; 
and silica, which is abundant in the wheat, is absent 
from the clover. Poisonous substances even may be ab- 
sorbed, if they are of such a nature as to be capable of 
absorption ; and so the plant may be killed by its own 
action—by suicide, as it were. 
The entrance of water into the plant and the entrance 
of those soluble materials which a plant derives from the 
soil are therefore illustrations of the process of osmosis, 
and are subjected to all the conditions under which os- 
mosis becomes possible, or under which it ceases to act. 
The study of these conditions is a question for the physi- 
cist, and the full explanation of them must be sought in 
works relating to physics. So the investigation of the 
substances which are absorbed with the water, of the 
food materials, and their transformations within the 
plant, is the work of the chemist, and their history must 
be sought in chemical books. 
