600 The Outline of Science 



devours another, but in the long run animals depend on plants. 

 We see a new meaning in the saying "all flesh is grass." Secondly, 

 there is the fundamentally important fact that the oxygen of the 

 air, necessary to keep the fire of life burning, is produced by green 

 plants, which are able to split up carbon dioxide. In the original 

 atmosphere of the earth there was little or no free oxygen. 

 Thirdly, when we think of one of the greatest steps in evolution, 

 the colonising of the dry land by animals, we recognise that plants 

 prepared the way. Not only did green plants supply food and 

 oxygen ; they afforded shelter, concealment, and abundant oppor- 

 tunities for animal adventures. That animals have paid their 

 debts is plain when we think of the making of good soil by earth- 

 worms, or the pollination of flowers by their insect visitors. 



Importance of Minute Plants 



In some way, still imperfectly understood, green plants are 

 able to feed at a very low chemical level, on carbonic acid gas, 

 water, and salts. The energy of the sunlight, shining through a 

 screen of green pigment (chlorophyll), is utilised to dislocate the 

 carbon dioxide molecule, and to begin the upbuilding of carbon 

 compounds, such as sugar. This is the most important process 

 in the world, and is known as photosynthesis the upbuilding of 

 carbon compounds with the help of the energy of the sunlight. 

 On this depends all the potential energy in the sacks of wheat, in 

 the bales of cotton, in the fields of rice, and in the bodies of ani- 

 mals. The energy of the coal consists of the bottled sunshine of 

 distant millennia. 



It is important to form a vivid picture of the circulation of 

 matter in which green plants play so essential a part. The atoms 

 of Carbon, Hydrogen, Oxygen, and Nitrogen are continually 

 changing partners in the never-ceasing dance, except when they 

 sink into a resting group. The carbon dioxide formed in the 

 vital combustion of the animal's body and liberated in the breath 

 may be recaptured by the green leaf. The nitrogenous waste of 



