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



105 





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.) 



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 

 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, 0, 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, C4H8O3N2. 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 

 trr.iins, of which the 

 structure is rendered 

 visible by treatment 

 with reagents. 



