10 PLANT LIFE ON THE FARM. 
Fortunately there is, in general, no Jack of it ; the earth 
and the air contain their shares of this elementary com- 
pound in varying proportions and varying modifications 
as liquid or gaseous. Besides, the plant itself has so 
much of it that even at the driest condition compatible 
with life, it still constitutes a very large proportion of the 
entire weight. Now, it is as a rule when the plant, .the 
seedling, or the bud is at its driest that growth begins, 
the necessity for food first manifests itself, and the demand 
for a further supply of water becomes imperative. How 
is the demand supplied ? We have seen that there is no 
lack of that fluid. How is it to get into the plant? 
The answer to this question brings us at once to the con- 
‘sideration of the raw material and of the fabric of plants 
by whose agency alone it is that the water gains entrance 
‘to the plant. 
Ingress and Movements of Water; Diffusion, Osmo- 
sis. —Our first inquiry, then, must be to ascertain how 
the water, whose presence in sufficient quantity we have 
assumed, gets from without through the cell-membrane 
into the protoplasm—how, in fact, the first stage in inde- 
pendent nutrition is accomplished. When one liquid, 
say spirit, is poured into another, say water, the two 
gradually mix. » If we suppose these liquids to consist of 
‘a number of molecules,* then, mixture may be taken to 
*It may be well, once for all, to explain the sense in which the term 
‘molecule’? is here used. It is now generally assumed by physicists 
that every substance in nature is made up of excessively minute parti- 
cles called atoms, which are indestructible. An atom cannot exist by 
itself, but in association with others. Such a group of atoms is called a 
“molecule.” A molecule, therefore, is the smallest group of atoms 
capable of existing separately and independently. These molecules may 
be of different sizes in different cases, and they are believed to be so 
arranged as just not toteuch, but to leave spaces between them ; 
smaller in the case of a hard solid, wider m that of a liquid, still wider 
in that of a gas. The extent, moreover, of these interspaces may be 
-{nereased or diminished by varying degrees of heat, 
