26 FIELD CROPS FOR THE COTTON-BELT 



carbon dioxide is decomposed and various food materials 

 are elaborated. The greater part of the energy which 

 the plant secures from the sunhght, however, is expended 

 in the evaporation of water from the leaves. 



THE GIVING OFF OF WATER 



28. We have seen that the cotton leaf is an organ for 

 the reception of light and the absorption of gases. It 

 is also by means of the leaves that the cotton plant rids 

 itself of the large amoimt of surplus water absorbed by 

 the roots. Not all of the leaf area, however, can be classed 

 as transpiring surface. In fact, to prevent the too rapid 

 loss of water, the surface of the leaf is made water-proof 

 by waxes so that water can escape only at the stomata. 

 Each stoma is surroimded by two guard-cells which serve 

 as automatic devices for regulating the loss of water from 

 the plant. The following quotation from Osterhout makes 

 clear the function of the guard-cells. 



"When the water-supply is abimdant, especially in 

 the presence of sunhght, the guard-cells absorb water 

 and expand. The pressure causes the walls that bound 

 the pores or stomata to open. This is due to the fact that 

 these inner walls are thicker than the outer walls. The 

 effect is the same as would be produced on a rubber tube 

 by thickening one side by cementing an extra strip of rub- 

 ber on it. If such a tube be closed at one end while air 

 or water is pumped in at the other, it wiQ bend so that the 

 thickened side becomes concave. 



"The absorption of the water by the guard-cells is aided 

 in sunlight by the action of the chlorophyll grains which 

 they contain; these produce sugar which aids the cells 

 in taking up water from the other cells of the epidermis 

 that have no chlorphyll grains. 



