June, 1919] Variations in Transpiration 497 



The temperature is expressed in degrees Fahrenheit, and 

 the humidity, reduced to saturation deficit (100% — % of 

 humidity) is expressed in percentages. The evaporation is given 

 as the rate in grams per hour of water loss from the Standard 

 Cup, calculated from the coefficients furnished by the Plant 

 World Company. Duration of sunshine is expressed in per cents 

 per hour of possible sunshine. Transpiration is given as the 

 rate in grams per hour of water loss from one hundred square 

 centimeters of leaf area. Leaf water contents are reduced to 

 leaf water deficits by substracting the figures for each from 

 the maximum water content during the night, and are ex- 

 pressed in percentages. The areas of the stomatal pores and 

 the peripheries of the stomatal pores are expressed in square 

 microns and microns respectively. 



Experiments. 



In Experiment 1 the leaf water contents and stomatal 

 movements of tobacco and mullein were measured, together 

 with transpiration and the several environmental factors. 

 These make possible a comparison of the effects of the various 

 factors. 



Five mullein ( Verbascum thapsus) and five tobacco {Nico- 

 tiana sp.) plants were used in this experiment, one of each in 

 determining the transpiration, and the other four in estimating 

 the leaf water content and stomatal measurements. The plants 

 were all sealed and irrigated, and were given special care for a 

 week before they were used, and during the experiment they 

 were exposed to the same conditions. 



The apparatus for determining the transpiration was auto- 

 matic and needed very little attention, while the leaf water 

 content and stomatal measurements required hourly observa- 

 tions. The method of making these observations was to strip 

 small pieces of the lower epidermis from the leaf, put them at 

 once in absolute alcohol, and then cut the leaf from the plant and 

 place it in an air tight weighing bottle. The leaves were 

 weighed at once and dried to constant weight and the water 

 content calculated from the results. The pieces of epidermis 

 were mounted on a slide in absolute alcohol and outlines of a 

 representative number of pore openings drawn on paper with 

 a camera lucida. Later these outlines were measured and 

 reduced to their actual size in microns. A polar planimeter was 



