46 INTRODUCTION TO BOTANY 



and consume part or all of it in the growth of the following 

 spring. Trees and shrubs in temperate or cold climates store 

 starch and other foods in the roots, as well as in the stem. 

 It is the stored food in the root that enables such plants as 

 rhubarb, the peony, some buttercups, sweet cicely, the dande- 

 lion, and many others to make a quick growth in the spring, 

 before the weather is warm enough for the manufacture of 

 much plant food. The starch, sugar, and proteins which 

 abound in many roots or root-like portions of plants make 

 them valuable for food, as in the case of beets, turnips, car- 

 rots, parsnips, sweet potatoes, salsify, and the cassava plant, 

 from which tapioca is made. 



45. Digestion and assimilation. Plants as well as animals 

 must make solid foods into solutions before these foods can 

 become parts of the living protoplasm. The processes which 

 make these solids into solutions are known by the general 

 name of digestion. Digestive processes vary widely in different 

 living things. Animals usually have special organs for this 

 work, but plants do not have them. The process is essentially 

 the same in both, however, though it may be much more com- 

 plex in some higher animals than in plants. Plants form cer- 

 tain chemical substances known as enzymes (see sect. 157), 

 which operate in ways that are little understood, but which 

 result in digesting foods. 



By a process known as assimilation digested foods may be 

 taken into living protoplasm and made a part of it. What 

 occurs in assimilation no one really knows. Digested food 

 becomes living protoplasm, and in so doing it becomes more 

 complex in structure. Thus far it has been impossible for 

 scientists to follow the process closely enough to determine 

 the changes which assimilation makes, though it is known 

 that living substance is the outcome. 



46. Respiration. Food that has been digested and assimi- 

 lated, thus becoming living tissue, is later changed by the 

 process known as respiration. It is common to associate 

 respiration, in both plants and animals, with the interchange of 



