10 PLANT LIFE ON THE FAKM. 



Fortunately there is, in general, no lack 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 to touch, but to leave spaces between them ; 

 smaller in the case of a hard solid, wider in that of a liquid, still wider 

 in that of a gas. The extent, moreover, of these interspaces may be 

 increased or diminished by varying degrees of heat, 



