16 PLANT LIFE ON THE FARM. 
Continuous Changes in Plants.—In this place we must 
confine ourselves to the few passing references already 
made, but one thing we must strive to impress forcibly 
on the reader, because, if the notion is well grasped, it 
will enable him to understand plant life so much more 
vividly. We allude to the continual changes that are 
going on throughout the whole living fabric of the plant 
while in its active condition. Cell membrane, the pro- 
toplasm, the entire mass of liquid and solid constituents 
of which the plant consists, are, as we have seen, made 
up of molecules, each, as it were, with a life of its own, 
undergoing continual changes according to different cir- 
cumstances, acting and reacting one upon another, so 
long as any active life remains. Active life, indeed, is 
ceaseless change ; dormant life is a condition of equi- 
librium, more often talked about than realized—in fact, 
it is merely relative—it implies merely a lessened degree 
of activity. From this physical point of view the death 
of a cellis only a change, arearrangement of particles, 
never, however, to be recombined into a new growing 
cell, as happens in the case of a still living cell in the 
full tide of growth and activity. 
Nutritive Value of the Substances absorbed by Plants. 
—The importance of water may be judged from the fact 
that while succulent vegetables contain more than ninety 
per cent of water, timber felled in the driest time seldom 
contains less than forty per cent (Warington). 
As to the nature of the saline substances, reference 
must, as has been said, be made to the ‘‘ Chemistry of 
the Farm” and other works for full details. Suffice it 
here to say that certain of them, though always in rela- 
tively small proportions, are essential to the life of the 
plant ; certain others, generally met with, though useful, 
are not indispensable. The former comprise salts of 
potash, magnesia, lime, iron, and in addition phosphorus 
