344 Physiologie. 



measurement of stomata in situ is described. The method is adapted 

 to tield work by night and by day. 



Leaf water, stated in percentage of dry weight, was found to 

 Vary in the cotton plant under usual conditions between 318 and 

 220 per cent. On the day of Observation the minimum leaf water 

 content was reached at the 14 hour or there abouts. This reduction 

 represents a net loss as shown by the determinations made relative 

 to Unit area and, therefore, with quantitative regard to dry weight. 



The amount of loss of leaf water when thus determined is from 

 7 to 15 per cent on the initial amount at sunrise, under the condi- 

 tions prevailing when the observations were made. Among these 

 conditions it may be mentioned incidentally that the soil was well 

 drained and rieh in moisture at the time. Severer circumstances 

 would no doubt effect a still greater loss. The observed loss, how- 

 ever, may be taken as indicative of a usual phenomenon, the reality 

 of which is made evident in an observed daily wilting of the leaves 

 beginning at about the 9 hour, detectable not alone by change in 

 Position (since this may occur as a phototropic response) but by 

 flaccidity. The case may be otherwise stated by saying that under 

 usual day conditions, with sunshine, the roots are unable to 

 supply loss ot water from the leaves. Balls' view that the water 

 supply is the limiting factor of growth, and his Observation that 

 no growth takes place under the Egyptian sun appear to be quite 

 applicable to Alabama. With regard to the amount of grov^^th 

 in Alabama, preliminary measurements, prompted b}^ the results 

 obtained from the determinations of leaf water, indicate that 

 even under the presumably more favorable humidity conditions 

 obtaining here growth does not take place for the major portion of 

 the day, since during the latter part of the growing season an 

 actual daily shrinkage in stem and leaf length has been observed 

 The writer can hardly concur, however, with Balls in his view 

 that because growth does not take place in sunshine this is to be 

 interpreted as unfavorable. Comparative measurements on the same 

 variety of cotton obtained in Arizona betray a no more unfavorable 

 reduction of leaf water than in Alabama, when there is sufficient 

 water in the soil. There is evidence that variations in soil moisture 

 are registered in both the absolute leaf water content and in the 

 rate of recovery after the minimum quantum for the day has been 

 reached, while the loss during the first part of the day appears to 

 be less affected. It would seem that the real test is the growth integer 

 for the season, and it is not evident that, with irrigation, the con- 

 ditions in the semiarid Arizona desert at all events are unfavo- 

 rable from this point of view. A hot sunshiny day after all ma}^ be 

 good for cotton, but this good maj'- not be apparent in growth at 

 the time. This is indicated by the amount of photosynthates formed 

 (measured with small error due to well understood causes) by the 

 increase in dry weight. Two series in Arizona gave increases of 

 24 and 32 per cent for 8 hours. In Alabama, with the exception 

 of one series of old leaves in which there was no apparent change 

 in weight, the increases ranged from 6 to 25 per cent. the latter 

 being one instance out of a total of seven series. Whatevcr ma}^ be 

 said of increase in dimensions, therefore, it remains the fact, that 

 in spite of the hot unmodified Arizona sun shining throughout 

 continuousl}'' cloudless days in August, more energy in the form 

 of carbohydrates was made available than in the similar periods in 

 Alabama. It is proper, however, to recall that the leaves studied 



