1919] Coit-Hodgson: Abnormal Shedding of Washington Navel Orange 327 



closure is not complete and the stomata remain slightly open. Heil- 

 bronn"^ has establislicd this same condition in the leaves of the 

 Camclia. It is interesting to note in this regard the results obtained 

 by Shreve'* in a studj' of the transpiration of rain-forest plants carried 

 on in Jamaica. 



The true stomatal transpiration is thus found to be from 42 to 48 per cent 

 of the total water-loss of the leaf. The close relation of transpirational behavior 

 to evaporation is thus shown to have its basis in the fact that rather more than 

 half of the water-loss of the plant goes on through the epidermal surfaces. . . . 

 The amplitude of stomatal movement in rain-forest plants under shade con- 

 ditions has been found to be relatively small. . . . The weakness of the move- 



Fig. 8. Cross-section of stoma from old coriaceous orange leaf. Note resin- 

 ous deposit in the substomatal cavity. 



ments, together with the high cuticular water-loss, serves to give the stomata a 

 very negligible role as regulators of transpiration rate, particularly during the 

 daylight hours. 



It was found that a varying percentage of citrus stomata are 

 occluded by depo.sits of a resinous, gummy nature (fig. 8) in the sub- 

 stomatal cavity. ITaberlandt"' points out that pliysiological degenera- 

 tiiin of stomata takes place in a nuiiiber of shade-loving hygrophytes, 

 doubtless because members of these ecological classes never require 

 much protection against excessive transpiration. Therefore it can be 

 readily appreciated that the citrus plant has relatively little control 



osBcr. d. dcut. bot. Ges., vol. 34 (1916), pp. 22-31. (Cited from Exp. Sta. 

 Record.) 



•x The Transpiration Behavior of Rain-forest Plants, Ann. Rep. Dcpt. Bot. 

 Res., Carnegie Inst. Washington, YearbooU 12 (1913), [ip. 74-76. 



05 Physiological Plant Anatomy (London MacMillan, 1914), p. 272. 



