186 GENERAL BIOLOGY 



Photosynthesis. 



In the final analysis every particle of food an animal eats must come 

 from and through the plant world. For example, when man eats a piece 

 of steak, the animal from which it was taken lived either directly on 

 plants or on other animals which fed on plant-life. 



Here it is well to appreciate the interesting way Nature has of 

 keeping a sort of balanced quantity of all needed organisms, for the 

 meat-eating animals or carnivores do not allow an overproduction of, 

 plant-eating animals or herbivores, and are prevented from multiplying 

 too rapidly by parasites in their own ranks, while much of the vegetable 

 world is saved because animals eat each other. 



The thought of these facts has led to the statement that the impor- 

 tant thing in life was to get enough to eat and to-prevent one's self from 

 being eaten. 



The plants manufacture their own food from the substances they 

 can extract from the surrounding soil and the air. Plants are there- 

 fore not dependent upon other animals or plants for their food as ani- 

 mals are. Those organisms dependent upon other living organisms for 

 their food are said to be heterotrophic ( ) in nutrition 



while those which can manufacture their own food are said to be auto- 

 trophic ( ) in nutrition. 



But only those plants which possess' chlorophyl are autotrophic. 

 Therefore fungi, molds, and most bacteria, which are plants, but which 

 have no chlorophyl, are heterotrophic; and, being obliged to live upon 

 other organisms, they are parasitic ( ) or sapro- 



phytic ( ). 



Chlorophyl is either contained in a chloroplast, as already stated, or, 

 in the simplest form of green plants, it is scattered throughout the pro- 

 toplasm. Chemically "chlorophyl is a complex compound of carbon, 

 hydrogen, oxygen, nitrogen, and magnesium ; its probable empirical 

 formula is given by one investigator as C r , 4 H 72 O B N 4 Mg. While not 

 a constituent of chlorophyl, iron is always present in the chloroplast 

 and seems to be essential to chlorophyl formation. Either in solution 

 or in the living plant chlorophyl absorbs part of the light which falls 

 upon it." The energy of the light thus absorbed by the chloroplast is 

 what enables the plant to perform its work. As light is required this 

 process goes on only during the day. 



"The materials from which carbohydrate food is manufactured by 

 green plants are two in number, carbon dioxide and water. Carbon 

 dioxide is present in the atmosphere in the small but constant concen- 

 tration of about 3 parts per 10,000 parts of air, and is therefore readily 

 available to such plants as the Pleurococcus. Water is absorbed directly 

 from the substratum through the cell wall into the protoplast. The car- 

 bon dioxide taken in is dissolved in the water in which it is readily 

 soluble. While the exact steps in the process of formation of carbohy- 



