12 FRANCIS E. LLOYD 



transpiration would at the same time assist the leaves in increasing 

 their water-content. 



Whatever may be the true explanation, there is a high probabil- 

 ity that the loss of water from the leaf in the morning is approxi- 

 mately a measure of the excess of outgo over intake of water as 

 indicated by the gravimetric and volumetric data. It is scarcely 

 less probable that the reverse relations of the afternoon obtain 

 under normal conditions, but a proof of either of these, in a strict 

 quantitative sense, is still to be obtained. 



It is obvious that, in estimating the amount of leaf-water in 

 terms of dry weight, we are using a varying measure, increasing 

 in value during a part or all of the day, due to photosynthesis. 

 This would show a decrease in the relative amount of water, but, 

 unless we assume that there is a decrease in the value of the meas- 

 ure (due to a loss of photosynthetic materials) after about midday, 

 we cannot explain the rise in water content after that time except 

 on the inference of an increase considerably in excess of the 

 amount at sunrise. 8 That is, with a constant increase in dry 

 weight, there should be a constant decrease in the relative amount 

 of water. Data now on hand and in process of publication show 

 that, in the cotton plant, there is generally an increase in dry 

 weight, until mid-afternoon, at any rate, and probably until sun- 

 set. Should it develop that the same is true of the ocotillo, the 

 inference would then be clear. It was my purpose in the summer 

 of 1911 to make the determinations of dry weight and leaf -water 

 per unit area of leaf in the ocotillo, but at the time of my visit, 

 it had not sufficiently developed its leaves and I am therefore 

 compelled to await further opportunity. 



Finally it is of interest to note the behavior of the stomata. My 

 conclusions at this time are based upon the measurements, kindly 

 made for me bj r Mr. C. S. Ridgway, of 1800 stomata, 300 for 



8 Fitting observed that the leaves of desert plants in the region of Biskra showed, 

 in spite of the high osmotic pressures observed in them by him, a water content 

 much below their capacity (Die Wasserversorgung und die osmotischen Druck- 

 verhaltnisse der Wiistenpflanzen. Zeitschr. f. Bot. 3: 209-275. 1911, p. 258). 

 Whether the ocotillo exhibits the same condition remains to be seen, but it seems 

 unlikely that under the conditions of the experiments here described, the water- 

 content of the leaves in the afternoon would exceed that at sunrise. 



