Jan., 1920] Resistance of Leaves to Transpiration 61 



The transpiration rate was determined from plants, (Fig. 2), 

 which were growing in six inch pots at the time they were used. 

 In every experiment plants as nearly alike as possible in size 

 and color were chosen. The potted plants were placed in six 

 inch aluminum shells and sealed with paraffined cloth, cut to 

 fit the top of the pot and shell, with a slit along the radius to a 

 hole in the center for the plant stem. Small paraffined cloth 

 washers were placed closely around the stem and carefully 

 made air tight with warm paraffin. The six inch pots did not 

 fit down in the shell, but rested on the edge, which left an air 

 space between the bottom and sides of the pot and the shelL 

 This space insured aeration of the soil as well as perfect drainage. 



The matter of maintaining a uniform amount of moisture in 

 the soil was at first done by watering the plants every 24 hours 

 with as much water as they had lost during that time. Later 

 porous cup irrigators were obtained which kept the soil in a 

 six inch pot in perfect condition. The method of operating 

 the irrigators was to seal a T-tube in the open end and fill it with 

 water and insert it in a hole in the pot made by taking out a. 

 core of soil. A small piece of soft rubber hose was put on the 

 straight end of the T-tube, with a pinch cock to close it and the 

 arm of the T-tube was connected with a reservoir which was. 

 arranged so that the water level was below the pot. 



In order to compare the transpiration rates of several differ- 

 ent plants a very careful record of the leaf areas was kept and 

 the transpiration from equal leaf areas was used. The leaves 

 were measured by tracing their outlines on paper and measuring 

 their total area with a planimeter which was adjusted to read in 

 square centimeters. Leaf areas on every plant for every experi- 

 ment were taken before the apparatus was started and as soon 

 as the experiment was completed and as there was a considerable 

 increase in leaf area for this length of time some method of 

 taking an average had to be determined. By plotting the 

 areas on cross section paper, using time as abscissas and leaf 

 area as ordinates, and assuming that the increase was a straight 

 line between those two points, the area represented at noon was 

 taken as the leaf area for that experiment. All results were 

 reduced to the same area basis. 



These experiments were performed in the research room of 

 the greenhouse and in a dark room adjoining the Plant 

 Physiology Laboratory. The dark room is light proof and has. 



