NUTRITION AND SEASONAL LIFE OF PLANTS 125 



digestion takes place in the cells of the storage endosperm or in 

 the embryo itself, where reserve foods were laid down during 

 the growth of the seed from the ovule. Digestion may therefore 

 be said to be distributed throughout the plant body, and for the 

 most part takes place within the cell cavities in the higher plants 

 instead of being localized in a special digestive tract. 



The agents of digestion are the so-called enzymes, or ferments, 

 familiar to us in the secretions of the digestive glands of man, 

 such as the saliva, gastric juice, and intestinal secretions. In 

 plants these ferments are usually formed by the protoplasts in 

 the cells where digestion goes on (Fig. 63) ; but in some seeds, 

 like the grasses and cereal grains, special glandular layers of 

 secreting cells exist where digestive ferments are formed and 

 then excreted into neighboring tissue cells for digestive purposes. 



The method of digestion is chemical in nature and consists, in 

 most cases, in adding water to the insoluble food molecules, thus 

 rendering them soluble, as in the conversion of reserve starch 

 to sugar: 



Starch + water -f diastase ferment = sugar + diastase 

 (C 6 H 10 6 ) n H 2 (C 12 H 22 U ) 



In this process, which is termed hydrolysis, the exact r61e of 

 the ferment is unknown, since it is not apparently destroyed or 

 diminished by the process, as indicated in the formula. 



ASSIMILATION 



The conversion of foods rendered soluble by digestion into 

 the living protoplasm of plant cells is termed assimilation. Since 

 this conversion is impossible unless the food is in the proper 

 state, digestion and assimilation are closely linked processes in 

 plant nutrition. 



SEASONAL LIFE OF ANNUALS, BIENNIALS, AND PERENNIALS 



We shall now attempt to apply to the seasonal life of a few 

 common plants the principles of nutrition already laid down. 



In order to make this application as comprehensive as possible, 

 plants will be selected which live under quite different conditions 



