ELABORATION OF FOODS INTO PLANT STRUCTURES 95 
from the place where they entered the dissolving medium, and 
thus toward those regions where they are less concentrated. In 
case a number of substances are in solution at the same time, each 
diffuses independently of the others. When, for example, sugar, 
salt, and ink are dissolved in a vessel of water at the same time, 
each diffuses to all parts of the vessel independently of the others 
and, consequently, the substances become thoroughly mixed just 
as the oxygen, nitrogen, carbon dioxide, and other gases of the 
air by diffusion tend to thoroughly mix. It is apparent then in 
case of the seed that foods in a concentrated solution in the endo- 
sperm or cotyledons will diffuse to the radicle and plumule, where 
the food, by being constantly removed from the solution to be 
built into plant structures, is kept less concentrated. 
Osmosis mentioned as another process involved in the trans- 
portation of foods is also a diffusion, but differs from the ordinary 
diffusion just described in that it takes place through a membrane 
which alters the rate of the diffusion of different substances by 
allowing some to pass through it more readily than others. It is 
by this kind of diffusion that substances pass into and out of liv- 
ing cells, in which case the membrane through which the sub- 
stances must diffuse is the modified border of the protoplasm. 
Thus, although foods depend much upon ordinary diffusion for 
transportation when not passing through membranes, in entering 
or leaving living cells they must also depend upon osmosis, the 
nature and principles of which are more thoroughly discussed in 
connection with the cell (Chapter VII). 
The Elaboration of Foods into Plant Structures. — In the early 
stages of germination the radicle and plumule elongate by the elon- 
gation of the cells already present, but soon, however, in certain 
regions, mainly at or near the tip of the radicle and plumule, there 
begins cell division followed by elongation, growth, and forma- 
tion of tissues — the processes upon which the continued develop- 
ment of the young plant depends. Throughout these processes 
foods are elaborated: (1) into materials to thicken the cell walls 
as they become thinner in stretching; (2) into protoplasm which 
must increase as cells grow and divide; (8) into woody and other 
elements for strength and conduction; (4) into fatty and waxy 
substances and cell thickenings for protection; and (5) into the 
various materials which are peculiar to food-making, reproduc- 
tive, absorbing, secreting, and other structures which plants form 
