PHYSIOLOGY OF CORN PLANT 47 



ASSIMILATION 



37. The taking up of carbon from air and uniting it 

 with other elements to form plant tissues is called assim- 

 ilation. Carbon is found in nature, as coal or graphite, 

 or it is artificially prepared from wood, as charcoal. 

 Carbon is the most important constituent of all plants, 

 composing about 50 per cent of the dry weight. 



Carbon can be demonstrated by charring, that is, by burning 

 a piece of maize stem without sufficient air for complete com- 

 bustion, when other substances will be driven off by the heat, 

 leaving the carbon. So much carbon is present that the stem 

 wiU retain its shape and structure. 



When any substance is burned or decomposes, the 

 carbon present passes into the air as carbon dioxid (CO2) . 

 This gas constitutes about 0.03 per cent of the atmos- 

 phere. 



A maize plant takes air into the leaves through the air 

 I^ores (stomata) and extracts the carbon dioxid. The 

 air then passes out again, carrying water and by-products 

 — ■ often oxygen — of which the plant should rid itself. 



38. The necessary energy for maintaining the activities 

 of the leaf is derived from the sunhght. Some of the 

 leaf cells contain small green chlorophyll bodies. When 

 the plant is in strong sunlight, these chlorophyll bodies 

 rapidly accumulate starch grains. If the plant is placed 

 in darkness, however, no starch will be made. 



In the same way, we may show the necessity of carbon dioxid, 

 by placing growing plants in air artificially freed of this gas. 

 Even in the presence of bright sunshine, no starch will be accu- 

 mulated. 



39. The by-product of assimilation is pure oxygen. The 

 chemical process of the manufacture of starch from carbon 

 dioxide and water, through the activities of chloroplasts. 



