INTRODUCTION 



25 



bination. If we ate only carbohydrates and fats> which contain 

 no nitrogen, and in addition some nitrates or ammonium salts, 

 we not only would not grow, but would gradually starve to 

 death because of our inability to build up the complex nitrogen- 

 containing compounds necessary for the building of new cells 

 and for the replacement of old 

 cell parts. What are the sim- 

 plest forms and the smallest 

 number of kinds of organic ni- 

 trogen-containing compounds 

 necessary for the building of 

 new cells and for the replace- 

 ment of old cell parts? What 

 are the simplest forms and the 

 smallest number of kinds of 

 organic nitrogen required for 

 animals? This question is a 

 very important one. It is like 

 asking what sort of bricks are 

 necessary for constructing a 

 certain kind of house. The 

 animal must be supplied with 

 the right kind of bricks, the 

 green plant makes it own. 



Even though sufficient food 

 of the right kind, together 

 with water and mineral salts, 



are supplied, growth will be markedly interfered with unless 

 the food contains those interesting accessory food materials, the 

 vitamins. Of unknown chemical composition and needed in very 

 small amounts, they are likely to be overlooked, with unfortu- 

 nate effects on growth. Most plants seem to construct their own 

 vitamins and they are therefore directly or indirectly the source 

 of the vitamins for animals. In some cases traces of organic mat- 

 ter have a marked effect upon the growth of green plants. 



Figure 15. The effect of long days and short 

 days on the growth of the coneflower 

 (Rudbeckia). Seed sowed December 19. The 

 plant at the left received, in addition to day- 

 light, electric light from sunset to midnight. 

 The plant at the right received 7 hours' 

 illumination only. Photographed May 14. 

 After Garner and Allard. Courtesy of the 

 United States Department of Agriculture. 



