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 
