TWO PATTERNS OF LIFE 15 



different ways of securing food during constructive metabolic 

 activity. Plants have perfected the autotrophic type found in 

 primitive fashion among the sulphur and iron bacteria, manu- 

 facturing food out of inorganic substances absorbed from the 

 environment. Animals have lost this ability (assuming that they 

 once had it) and have specialized in heterotrophic metabolism, 

 i.e., a dependence upon ready made organic compounds as food. 

 This is ingested rather than absorbed, usually through a mouth 

 opening; after ingestion it is digested and made fit for absorption 

 and assimilation into animal protoplasm. This characteristic 

 makes all animal life dependent upon such autotrophic organisms 

 as green plants, which are the alchemists living on the border 

 between the non-living and the living world, capable of continu- 

 ally transforming substances from the former into organic materi- 

 als characteristic of life. 



Perfection of autotrophic metabolism has resulted in an 

 ability to tap far greater reserves of energy than those released 

 by oxidizing sulphur or iron. A vast amount of energy continu- 

 ally reaches the earth in the form of sunshine. Even though man 

 has long known of the billions of potential horsepower which 

 thus daily reaches us in the form of ordinary sun's rays, he has 

 been unsuccessful in constructing solar engines to capitalize this 

 power. The autotrophic green plants are able to harness this 

 immense stream of energy, by a process which seems relatively 

 simple. There appeared, sometime during the early evolution 

 of plants, a green pigment, chlorophyll, chlorophyll is com- 

 posed of carbon, hydrogen, oxygen, nitrogen and magnesium; it 

 is without doubt the most important compound known to man. 

 Produced by plant protoplasm in the presence of sunlight, when 

 an integral part of living protoplasm it acts as a screen to inter- 

 cept solar radiation. With this energy captured during daylight 

 hours the cells can carry out the difficult feat of rearranging the 

 atoms of water and carbon dioxide, which is a necessary step 

 in the synthesis of food. Chlorophyll may have been, at first, 

 diffused through the cytoplasm as is the case with the present- 

 day Blue-green Algae (see p. 86). But in time cellular specializa- 

 tion resulted in the localization of the chlorophyll in special 

 plastids, the chloroplasts. These chlorophyll-bearing structures, 



