PRINCIPAL FUNCTIONS 
541 
leaf or stem of a plant must also be digested before it can be 
absorbed. Enzymes play an important part in shortening 
the time that it takes to change foods into solutions. 
Review the following: pages 5, 13, 14,15; sections 114, 
120, 141, 206, 237, 246, 268, 326, 328; and figure 167. 
d. Assimilation. — After food has been digested, it is 
absorbed. In the paramecium or yeast cell, it is taken at 
once into the living protoplasm. There are no special 
structures such as are found in all complex organisms. The 
food is distributed throughout the cell. This simple condi¬ 
tion is not adequate when organisms are made up of thou¬ 
sands of cells and special methods of distributing the absorbed 
food are found. 
e. Circulation. — The organs of circulation in higher 
organisms may well be compared to our transportation 
systems, the railroads. Their chief function is to carry 
food, oxygen, and bodily wastes from one part of the body 
to another. In animals this system is known as the heart, 
arteries, capillaries, and veins; and in plants, as the fibro- 
vascular system of veins. 
Review the following: sections 48, 110, 114, 120, 141, 233, 
236, 237, 247, 248, 329, 357; and figures 64, 235, 256. 
/. Respiration. — Oxygen furnishes a necessary amount 
of energy to all organisms. Animals that live on land have 
special organs, the lungs, that are adapted to taking oxygen 
from the air; while aquatic animals, like fish and crayfish, 
have gills that are adapted to take oxygen from the water. 
Some animals, like the earthworm or hydra, have the cells 
covering the body adapted for this work. Plants have their 
leaves modifed by stomata, and through these oxygen passes 
into the plants. Under some conditions, the stem takes up 
oxygen. Aquatic plants, like algae, take up oxygen through 
the walls of the filament. 
Review the following: pages 5, 10, 15; sections 6, 46, 59, 
72, 115, 121, 142, 164, 175, 207, 237, 353; and figure 64. 
