118 THE PRINCIPLES OF AGRICULTURE 



their air by too much water, are soon suffocated. 

 This is especially noticeable in a field of Indian 

 corn or maize which has been overflowed ; and it 

 is also a condition frequently met with in those 

 greenhouses where an abundant use of water is 

 the first rule. Many plants which have become 

 accustomed to boggy regions, and many green- 

 house plants, send up to the surface numerous 

 root formations in response to a need of fresh 

 air, or oxygen. 



2d. Carbon dioxid and sunlight 



197. The element that is present in greatest 

 amount in plants is carbon. This material is 

 derived in green plants from the carbon dioxid 

 (or carbonic acid gas) of the air. 



198. In order to become plant-food, the car- 

 bon dioxid of the air first diffuses into the leaves ; 

 then its utilization depends on the green color- 

 ing matter of leaves,— or the chlorophyll, — and 

 on sunlight. The chlorophyll absorbs some of 

 the energy of sunlight, and by means of the 

 energy thus provided, there is effected a rear- 

 rangement of the atoms of carbon dioxid and 

 water, such that sugar, and ultimately starch, 

 may be produced and some oxygen is set free. 

 This process of the formation of plant-food from 

 carbon dioxid and water, with the consequent 

 giving off of oxygen, is photosynthesis (some- 



