8 FEEDS AND FEEDING, ABRIDGED 



while carbon dioxid and water contain but little. Therefore, to make 

 sugar and starch from these two energy-poor substances the plant 

 must secure energy from some outside source. This it obtains from 

 the sun, as light, which is absorbed by the leaves. 



The carbohydrates. — Sugar and starch, together with the related 

 products, the celluloses and pentosans, are called carbohydrates. This 

 group of plant compounds makes up the major portion of all plant 

 substance. The term carhohydrates means that these compounds are 

 formed of the three elements, carbon, hydrogen, and oxygen, the latter 

 two being present in the proportion existing in water, the chemical 

 formula for which is H^O. (This means that every molecule of water 

 contains two atoms of hydrogen and one atom of oxygen.) 



The molecular composition of the leading plant carbohydrates is as 

 follows : 



Glucose 1 p TT n 

 Fruit sugar J '"«^i^'-'« 



Cane sugar 1 p tt n 

 Malt sugar J ^i^^^^v^" 



CeUulose } (CeH.oOJx 



Pentose C^H^oOs 



Pentosan (CsHgOJx 



The molecules in the bracketed groups are in reality far more com- 

 plex than the formulae indicate, the actual molecule being many mul- 

 tiples of the groups here given. 



All sugars, not only the simpler glucose and fruit sugar but also 

 the more complex cane sugar and malt sugar, are soluble in the juices 

 of the plant. They are thus the portable, carbohydrate building ma- 

 terial of plants, which is carried in the sap to all their parts as 

 needed. Some plants, as the beet and the sugar cane, store their re- 

 serve food chiefly in the form of sugar. 



Starch is more complex in structure than the sugars and is in- 

 soluble in water. It is the form in which most plants chiefly store 

 their reserve food. This carbohydrate abounds in nearly all seeds, 

 forming over 70 per ct. of the dry matter in corn and wheat grains. 

 Often starch is stored in the underground parts of plants, as in the 

 potato tuber, or in fruits, as in the apple. Since starch is insoluble 

 in the sap, it must be changed into sugars by an enzyme or ferment 

 when it is needed in other parts of the plant. (See Page 22.) 



Cellulose is the ^reat structural substance of plants-, for the walls 



