12 BULLETIN 1018, IT. S. DEPARTMENT OF AGRICULTURE. 



ment " that shedding- occurs when, from any cause whatever, the 

 amount of water taken in by the roots falls short of that given out by 

 the leaves." Ewing (12) reports from his observations that " it 

 appears that the water content of the plant must have an important 

 influence on shedding, and the water content is, of course, dependent 

 on the amount of water absorbed by the roots and the amount tran- 

 spired." Lloyd (19) concludes that " the water deficit is the cause 

 of rises in temperature in the tissues, and this constitutes the stimu- 

 lus which directly leads to abscission." Balls (3) has observed that 

 " though the primary cause of shedding in Egypt is a deficient root 

 absorption, it follows that an excessive transpiration rate must 

 produce the same result, since the terms ' deficient ' and ' excessive ' 

 are relative." 



SHEDDING STUDIES UNDER ARIZONA CONDITIONS. 



From the observations of the writer it appears that under Arizona 

 conditions the severe water stress exhibited by plants possessing large 

 areas of leaf surface and the shedding of squares and bolls are not 

 always induced by a lack of " available " moisture in the soil. Such a 

 conclusion may seem paradoxical in the light of general opinion, but 

 to one who has studied the water relations of the cotton plant when 

 grown under arid conditions it may not appear unreasonable. It is 

 well known to plant physiologists that some plants are better pro- 

 vided than others with special structures for resisting too great a loss 

 of water when growing under arid or desert conditions. The cotton 

 plant seems to be one of the plants not so well equipped for reducing 

 evaporation under arid conditions and frequently loses more water 

 by transpiration than the roots can absorb from the soil. Recent 

 work on the relations of plants to water supply, including contribu- 

 tions by Lloyd (18), Livingston (17, p. 216), Free (13, p. 114), Lyon 

 and others {20), Coit and Hodgson (11), Balls (3), and others, had 

 indicated that a feeding rootlet, by absorbing some of the soil solu- 

 tion with which it is in contact, creates a condition of instability 

 which results in considerable film movement toward the drying-out 

 zone. Undoubtedly it is the deficiency of this assumed property of 

 capillarity in supplying the film moisture at a sufficient rate to re- 

 place the amount absorbed by the roots that accounts for the occur- 

 rence of water deficit in the tissues of the cotton plant, as shown so 

 clearly in large plants with a great deal of transpiring surface. It 

 is fortunately true that this deficit, occurring during the hours of 

 hottest sunshine, is largely made up during the late afternoon and 

 night, provided climatic conditions are not too stringent. When 

 very dry hot weather occurs at a time when the soil conditions are 

 unfavorable to the ready absorption of water by the roots, the 

 shedding of immature fruits reaches its maximum. 



