CH. Ill, 13] USES OF THE PLANT'S FOOD 



105 



certain chemical classes, the chief of which are the ALBUMINS, 

 material like white of egg, GLUTELINS, in semi-crystalline 

 grains (Fig. 66), GLOBULINS, fa- 

 miliar in the gluten of flour 

 which gives tenacity to dough, 

 NUCLEO-PROTEINS, the chemical 

 basis of the chromosomes (the 

 most important part of the pro- 

 toplasm), and a great many 

 others. While ordinarily in 

 solid grains, they are all digest- 

 ible by enzymes into soluble 

 and diffusible forms called PEP- 

 TONES and PROTEOSES, and thus 

 can be moved through the plant. 

 Chemically they are all very 

 complex, for to the elements of 



FIG. 65. Section across a 

 grain of wheat, showing the layer 

 of protein-holding cells under the 

 husk and outside of the starch- 

 holding cells ; X 180. (From 

 Strasburger.) 



grape sugar there are added small amounts of nitrogen, sul- 

 phur, and phosphorus, taken with water through the roots ; 

 and it is for this reason that nitrates and phosphates in par- 

 ticular are so essential to fertility in a soil. The stages in 

 their formation are complicated, and 

 only partially known, but it seems clear 

 that first the nitrogen is added chemi- 

 cally to the elements of the sugar, 

 forming amino-compounds or amides 

 (containing C, H,,O, N), with which 

 later the other elements are combined. 

 These amides are inconspicuous sub- 

 stances though widely distributed in 

 plants, the most common being Aspar- 

 agin, C 4 H 8 03N2. There is good reason 

 to believe that many of the proteins are 

 built up from a simple combination in 

 much the same way that we found the starches and cellu- 

 lose are based on a C 6 Hi O 5 foundation (page 98). These 



FIG. 66. A cell 

 from Castor Bean, 

 showing the protein 

 grains, of which the 

 structure is rendered 

 visible by treatment 

 with reagents. 



