122 



PLANTS AND MAN 



the Paleozoic Era. Thus plants form a link between us and the far 

 distant past. Coal is a more efficient fuel than wood since the 

 water which interferes with complete combustion of wood has 

 been removed in coal by the process of compression. 



In more recent times another fuel substance has risen to 

 importance. Petroleum, like coal and wood, is a compound of 

 carbon and hydrogen, and therefore combustible. The exact 

 origin of petroleum is still unknown, but one quite possible 

 explanation is that petroleum deposits are the result of the 

 accumulation of plant remains containing reserve oil as a storage 

 material. Such plants are the diatoms (fig. 80), whose cells con- 



FiG. 80. — ^Diatoms, minute floating algae, are considered to be a source of 

 some petroleum deposits. 



tain small globules of oil. When these microscopic plankton 

 organisms die, they sink to the bottom of the sea and escape 

 decay under conditions similar to those found in the coal-forming 

 swamps. But in this case the potential energy, instead of being 

 locked up in cellulose, is in the oily compound known today as 

 petroleum. Thus when we use gasoline to propel our automobiles, 

 we may be releasing solar energy which a half a billion years ago 

 filtered through some prehistoric sea, to be caught by tiny 

 diatom plants. If petroleum originated in this way, then prac- 

 tically all of man's power is derived from solar energy intercepted 

 by chlorophyll-bearing plants and stored in the organic com- 

 pounds which have resisted decay. 



Plants, like animals, produce secretions in special cells or 

 groups of cells known as glands, and also give off waste products 



