134 BIOLOGY 



in the darkness than in the light. As a result, green plants in 

 sunlight and in the daytime give off a surplus of oxygen, while 

 in the night they are giving off carbon dioxid but no oxygen. 

 Oxygen gas is a material that is utilized by animal life, while 

 carbon dioxid gas is a waste product of animals as well as plants. 

 Hence it has been said that, in the daytime plants are useful in 

 a living room, while in the night-time they are harmful. There 

 is really no foundation for this claim, since the amount of carbon 

 dioxid given off by a few plants in a room is so slight that it is 

 of no practical significance in its bearing upon animal life. In 

 nature, however, the plant and animal life balance each other; 

 while animals absorb the oxygen given off by plants, they them- 

 selves give off carbon dioxid that is utilized by plants; and thus 

 the condition of the atmosphere is kept practically constant 

 so far as concerns its content of both oxygen and carbon dioxid. 



In general, plants manufacture far more starch than they 

 need for their own life. The surplus is stored in some form as 

 starch, sugar, fat, proteid, or some other material, and upon this 

 surplus the whole animal world is nourished. 



All ordinary green plants carry on this process of photosyn- 

 thesis. Fungi, illustrated by bacteria, yeasts, molds, mushrooms, 

 etc. (Figs. 32, 34, 42), all agree in lacking the green chlorophyll 

 and are for this reason sometimes called colorless plants. Since 

 they have no chlorophyll they are unable to carry on the process 

 of photosynthesis, unable to utilize the energy of sunlight and 

 manufacture starch. But they must have energy as well as 

 green plants for their life, and are therefore dependent upon the 

 latter for their food. The Fungi are commonly found growing 

 and feeding upon organic foods, and are quite unable to utilize 

 the minerals of the soil and the gases of the air. They are 

 usually found, therefore, in the midst of masses of decaying 

 organic refuse, on dead tree trunks, in manure heaps, growing 

 from rotting leaves, etc. They feed upon the remains of past 

 generations of green plants, having, as we shall see later, a very 

 important part to play in nature's food cycle. 



