THE PLANT 29 



causing greater loss of water. If, however, seedlings or 

 young plants are allowed to become very short of water, 

 they may be limited thereby, with subsequent effects on 

 the crop, to which we shall later advert. 



The basis of these marked peculiarities of the plant in 

 regard to water is a structural one namely, the presence 

 of a rather exceptionally large number of " stomata " on 

 both sides of the leaf. These breathing-pores, which also 

 act as lip-valves for regulating the loss of water-vapour, 

 number about 300 to the square millimetre on the lower 

 leaf -surf ace (or 200,000 to the square inch), and 100 jm 

 the upper surface. In cottons of the Peruvian type the 

 surface is practically hairless, but in the Upland type the 

 leaf-surface is commonly hairy, and since the hairy tangle 

 prevents rapid motion of air past the exterior aperture 

 of the stomata, evaporation is diminished. Upland 

 cotton can in consequence endure dry weather with less 

 injury than Egyptians a fact which will have some influ- 

 ence on the development of cotton-growing in many new 

 areas. Some idea of the number of these apertures may 

 be gathered from the fact that a seedling without any 

 other leaves than the pair of original seed-leaves is 

 pierced with about a million stomata. The evaporation 

 of water takes place almost entirely through these aper- 

 tures in the older leaves, though in very young tissues, 

 which have not developed the impermeable skin of cuticle 

 properly, some water escapes directly from the skin of the 

 leaf. The stomata open and close in reaction to the con- 

 dition of the plant. If water is deficient, they close 

 before the plant is noticeably wilting, and so restrict 

 further loss. 



