218 The Living Plant 



digestive juices (diastase, pepsin) found in the alimentary system 

 of animals; for the solution or hydrolysis of insoluble foods by 

 enzymes constitutes digestion in plants just as truly as in animals. 

 This digested material is then in suitable condition for transporta- 

 tion, which takes place in two ways. First, it may be carried 

 with an onward-moving water current, as happens with the sap 

 in the spring (witness the Sugar Maple), when the food stored 

 for the winter in the roots or lower trunk of the tree diffuses from 

 the storage cells into the sap current and rises therewith. Sec- 

 ond, it may travel by diffusion alone, for a substance dissolved 

 in water is in perfect physical condition for diffusion, that 

 is, has the power and the tendency to move outward and on- 

 ward, by its own diffusive energy, from places of greater to 

 places of lesser concentration until equilibrium is established. 

 When, furthermore, the substance is being produced at one place, 

 as occurs with sugar in the leaves during photosynthesis, and is 

 being removed in another, as occurs in places of storage where it 

 is converted into insoluble starch, then a steady diffusion current 

 is established between the place of production and the place of 

 use. And it is by such diffusion currents that most of the trans- 

 location of food-substances through the plant is effected, though 

 it is to be remembered that diffusion alone, from its very nature, 

 can never completely empty a part. This explains why some 

 sugar and other food materials remain in autumn leaves when 

 they fall. 



This translocatory diffusion proceeds in part from cell to cell 

 through the walls, the protoplasmic linings thereof being adjusted 

 (by appropriate chemical modification or intermicellar spacing, 

 as noted earlier under Absorption) to permit the passage of the 

 molecules of the substance; and, given time enough, there is no 

 limit to the distance that substances may thus pass in solution. 

 Obviously, however, such translocation through long distances 

 must be greatly facilitated if long tubes replace the short cells; 

 and such a system is actually found in the elongated sieve-tubes 



