326 University of California Publications in Agricultural Sciences [Vol. 3 



the temperature of this upper layer of soil does not fall below 75° F. 

 As was brought out previously, under clean cultivation practices the 

 absorbing roots of citrus trees are largely located in the upper two 

 feet of soil (pi. 42). It therefore seems quite probable that during 

 the afternoon at the very period when water loss by transpiration is 

 greatest, absorption is inhibited by high soil temperatures. A study 

 of the cardinal temperatures for absorption by citrus roots, which is 

 expected to throw considerable light on this question, is now under 

 way and will be reported on later. 



But granted that a condition of physiological drought existed, due 

 to the action of the factors just discussed, still the citrus tree might 



Fig. 7. Citrus stoma showing maximum opening. From orange leaf just 

 reaching full size. 



maintain itself in a proper water balance were it not for the fact that 

 it is not provided with efficient means of conserving its water by 

 regulating its loss through transpiration. A preliminary study of the 

 relation of cuticular transpiration to stomatal water loss has brought 

 out the fact that from 40 to 50 per cent of the water loss from 

 citrus leaves occurs through the upper epidermis which does not con- 

 tain stomata. These studies have shown that the young leaves are 

 more efficient than the older leaves but that even the youngest leaves 

 lose as much as 25 per cent of their water through the upper epidermis. 

 A study of the stomatal condition in citrus leaves has brought out 

 some interesting facts. By the use of Lloyd's method 62 the amplitude 

 of stomatal movement was studied. It was found that very early in 

 the life of the leaf the stomata lose their power of opening and closing 

 and remain practically closed thereafter (fig. 7). In some cases the 



02 Physiology of Stomata, Carnegie Inst. Washington, Publ. 82 (1908), p. 26. 



