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 Uving 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 pliunule elongate by the elon- 

 gation of the cells already present, but soon, howeveK, 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; (3) 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 



