236 BRIEF OUTLIXE oF VEGETABLE I'llYslOLOGY 



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

 tlii'iMit;li 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 /t'r/He;i(.<. or enryH/e.^'. of which diastase is a common example. 

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

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

 germinating seeds (e. 7. barley). If a solution is apipdied to a bit of 

 starcli on a glass slide undei' the nncroscope, the disintegration of the 

 starcli granules may be observed.^ 



561. The formation of albuminous substances. — Assimilation is 

 oidy the first step toward the fornmtiou of living substance, or proto- 

 plasm. The albuminous substances which compose protoplasm dilfer 

 friHu the carbohydrates produced by assindlation, in containing a con- 

 sidi-ralile proportion of nitrogen often with some sulphur and phos- 

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

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

 final step in the production of pioteid matter is taken is not detiuitely 

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

 be at all growing pohits. 



562. The transfer of organic sidistanee, whether of carbohydrati's 

 or of lutrogenous compounds, is largely accomplished by the dilfusion 

 of snhuions of these substances. Albuminous matters not ditfusihle. 

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

 wdien the transfer takes place in a dicotyledonous steui.- This is the 

 route by which noirrishmeut designed for the root system is brought 

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

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

 arrest ami aceuirndatiou of nourishment in transit toward the I'oot. 



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

 needed for growth passes into the store of reserve luaterial. 



564. Living cells pei'form the olhce of storage. In stems and roots 

 these cells would be tho^e 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 commoidy in the form of 

 starch, but also in the foj-m of sugar. Ueserve cellulose is another 

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

 storage cells liecome greatly thirlceiieil by the depositions. Food may 

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

 ervsl ab. 



566. Respiration. — ,\li ]ilants. like all animals, take in oxygen. 

 -Vs plants are less active than ;iiiinials, they need less oxygen; and 



' See E,i::iiiiis, Siraslnirgpr. p. 20:1. 



- Ill the pliloeia of the librovascular bundles. For s>i_rr liihc-i see 

 Goodale, \>. 91. 



