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



105 



o 



Fig. 65. — Section across a 

 graia of wheat, showing the layer 

 of protein-holding cells under the 

 husk and outside of the starch- 

 holding cells ; X ISO. (From 

 Strasburger. ) 



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

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

 grains (Fig. 66 i, globulixs, fa- _ 



miliar in the gluten of flour 

 which gives tenacity to dough, 

 NUCLEO-PHOTEixs, 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 enzjTnes into soluble 

 and difJusible forms called pep- 

 tones and PROTEOSES, and thus 

 can be moved through the plant. 

 Chemically they are all A^ery 

 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 kno"«Ti, but it seems clear 

 that first the nitrogen is added chemi- 

 cally to the elements of the sugar, 

 forming amino-compomids or amides 

 (containing C, H, 0, X), ^ith which 

 later the other elements are combined. 

 These amides are inconspicuous sub- 

 stances though nidely distributed in 

 plants, the most conunon being Aspar- 

 agin, CiHsOjXo. There is good reason 

 to beheve that many of the proteins are 

 built up from a simple combination in 

 much the same w&y that we found the starches and cellu- 

 lose are based on a CeHioOs foundation (page 98). These 



\ 



Flu. bb. — A Lell 

 from Castor Bean, 

 showing the protein 

 grains, of which the 

 structure is rendered 

 visible by treatment 

 with reagents. 



