14 FIELD CROPS FOR THE QOTTON-BELT 



of Sea Island cotton are three-lobed also, but the lobes 

 are much longer and slenderer and the. indentations much 

 deeper than in the upland cottons. 



The principal fimctions of the leaves are: (1) to make 

 possible the free circulation of solutions of food and air 

 throughout the plant; (2) to give off the excess of water 

 taken up by the roots; (3) to take up from the air the 

 carbon dioxide needed to build plant-tissue; (4) to ^lab- 

 orate plant-food from the minerals and water taken from 

 the soil, and the carbon and oxygen taken from the air; 

 (5) to absorb from the sun the energy necessary for the 

 activities enumerated above. 



12. The vascular system. — In the description of the 

 cotton leaf attention was called to the system of leaf-veins, 

 ranging in number from three to seven. A careful exam- 

 ination will reveal a much-branched net-work of minor 

 veins springing from the larger veins. If a cross-section 

 of a leaf is examined imder the microscope, it will be seen 

 that these veins are composed of specialized tissue of 

 vessels and fibers. This fibrous tissue of the leaves extends 

 throughout the petioles, the branches, the main-stem, 

 and into the root-system, and is known as the vascular 

 system. It is by means of ,this vascular system that solu- 

 tions are carried from the roots to the stems and leaves. 



13. Air cavities. — Besides being supplied with food 

 and water, each leaf cell must have air, or rather carbon 

 dioxide from the air. To supply this there is provided 

 throughout the entire leaf tissue a system of continuous 

 openings, or air spaces, between the cells. These air 

 cavities communicate with the exterior in all the green 

 parts of the leaf. The openings through which the air 

 enters are known as stomata and are most numerous on 

 the under side of the leaves. By means of this delicate 



