236 BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 



form the newly made plant food in the cells of the leaf can pass out 

 through the petiole to the stem, and travel to points of active growth, 

 or to storage cells. Digestion is accomplished by means of the so- 

 called ferments, or enzymes, of which diastase is a common example. 

 The enzymes are not consumed in the process; their mere presence 

 seems to be enough to induce digestion. Diastase is extracted from 

 germinating seeds (e.g. barley). If a solution is applied to a bit of 

 starch on a glass slide under the microscope, the disintegration of the 

 starch granules may be observed. 1 



561. The formation of albuminous substances. Assimilation is 

 only the first step toward the formation of living substance, or proto- 

 plasm. The albuminous substances which compose protoplasm differ 

 from the carbohydrates produced by assimilation, in containing a con- 

 siderable proportion of nitrogen often with some sulphur and phos- 

 phorus. It is in the formation of these nitrogenous, or albuminous, 

 matters that the nutrient mineral salts are put to use. Where this 

 final step in the production of proteid matter is taken is not definitely 

 known. It may be that it is in the green tissue of the leaf, or it may 

 be at all growing points. 



562. The transfer of organic substance, whether of carbohydrates 

 or of nitrogenous compounds, is largely accomplished by the diffusion 

 of solutions of these substances. Albuminous matters not diffusible, 

 as well as solutions, are carried by the so-called sieve tubes in the bark, 

 when the transfer takes place in a dicotyledonous stern. 2 This is the 

 route by which nourishment designed for the root system is brought 

 from the leaves. If a ring of bark is removed from the trunk of a 

 tree, the bark above the cut grows and swells out, because of the 

 arrest and accumulation of nourishment in transit toward the root. 



563. Storage. Such a part of the elaborated food as is not at once 

 needed for growth passes into the store of reserve material. 



564. Living cells perform the office of storage. In stems and roots 

 these cells would be those of the bark, the medullary rays, and the 

 living pith. In tubers and other special organs of storage, the storage 

 cells are particularly numerous and often of large size. 



565. Carbohydrates are stored most commonly in the form of 

 starch, but also in the form of sugar. Reserve cellulose is another 

 storage condition of the carbohydrates ; in this case, the walls of the 

 storage cells become greatly thickened by the depositions. Food may 

 be stored in the form of oil and fat ; also in protein granules and 

 crystals. 



566. Respiration. All plants, like all animals, take in oxygen. 

 As plants are less active than animals, they need less oxygen ; and 



1 See Enzymes, Strasburger, p. 203. 



2 In the phloem of the fibrovascular bundles. For sieve tubes see 

 Goodale, p. 91. 



