540 GENERAL BIOLOGY — A REVIEW AND SUMMARY 
is the reason why plants have always preceded animals, as 
they have come to live first in one place, then another. For 
the same reason the quantity of food substances made by 
plants must always exceed the demand made upon them by 
animals, else the animals will starve. 
a. Food-getting in Plants. — Most plants get their food in 
the form of solutions through root-hairs and in the form of 
gases through the air. Root-hairs excrete substances that 
dissolve some otherwise insoluble materials and thus in¬ 
crease their food supply. Root-hairs are adapted to getting 
these solutions through their cell walls by osmosis. Leaves 
and young stems have stomata on their surfaces, which 
are openings that allow gases to pass through at certain 
times. 
Review the following: sections 241, 268, 273, 278; and 
page 306. 
b. Food-getting of Animals. — The simpler animals get 
their food in much the same way as root-hairs do, by simple 
osmosis. The higher animals have more elaborate structures 
for getting food. In the mammals, special organs, teeth, cut 
the food into smaller portions. It is then swallowed by 
muscles especially for that purpose and passed into the 
stomach. 
Review the following: sections 3, 5, 44, 58, 81, 99, 150, 
163, 174; and figures 3, 99, 106, 107. 
c. Digestion. — The food in the stomach of a mammal is 
acted upon by an enzyme secreted by the living cells lining 
the stomach cavity. Different enzymes grown in the living 
cells of the pancreas and stomach produce further changes in 
the food after it reaches the intestinal cavity. The action 
of these different enzymes is to reduce the foods to solution, 
which is digestion. The parts of the food that are thus 
digested are made soluble and pass by osmosis into the blood. 
Food in the food-vacuole of the paramecium is digested in a 
similar manner. The stored-up starch and protein in the 
