22 THE SMALL GRAINS 



perature within which photosynthesis may occur in the 

 small cereals is from just below freezing to 75 C., while 

 the optimum temperature is about 62 C. 



21. Protein production. The synthesis of nitrogenous 

 foods can apparently take place in any living portion of 

 the plant, but appears to be most common in the leaf; 

 and intermediate compounds or amides, such as asparagin, 

 are found there also in large amounts. Some of these 

 proteins contain carbohydrate groupings only, besides 

 nitrogen, while others contain sulfur also, and others 

 phosphorus. There is a great lack of definite information 

 on this subject, however, but it is known that protein 

 food construction goes on independently of the presence 

 of chlorophyll. 



22. Essential food elements. It is seen from the 

 foregoing that the primary elements entering into the 

 actual structure of the plant are carbon, hydrogen, oxy- 

 gen, nitrogen, and often sulfur and phosphorus. Analyses 

 have shown, however, that potassium, calcium, magne- 

 sium, iron, sodium, silicon, and chlorine occur in cereal 

 plants. All of these, except the last three, are said to be 

 essential elements. It is maintained that the presence 

 of potassium is essential in carbohydrate production. 

 Iron is necessary for the normal development of chloro- 

 phyll. Though silicon is very abundant in the ash of 

 cereals, it is not considered essential, because not required 

 in the growth of the plant. As it occurs in the cell wall, 

 it seems that it may, by giving support to the plant, be 

 of importance in plant adaptation. 



23. Provisions for food circulation. The seedling at 

 first has its food near at hand, which diffuses to the grow- 

 ing parts where needed through the rapidly forming 

 parenchyma cells. After exhaustion of the reserve foods, 



