384 MINNESOTA BOTANICAL STUDIES 



trol (Plate XLI). However, since all the plants in any determina- 

 tion were under similar conditions, the reader need not be burdened 

 with these data of factor measurements, and they will be omitted 

 except where they help to explain certain interesting results. 



The determination of the transpiring area was made as soon as 

 any of the leaves, which often kept an upright position throughout 

 the experiment, showed signs of drying at the tip. This occurred 

 from 8 to 14 days after the plants were sealed. It may be stated 

 here that this is a normal process and was not due to lack of mois- 

 ture, for at the end of the experiments the soil was usually still in 

 good tilth condition. At the conclusion of an experiment the plants 

 were placed in a very humid atmosphere until they were turgid, 

 then the leaves were clipped at the ligules and those of each con- 

 tainer placed together. Photographic prints of the leaves were then 

 made, using solio printing paper. The white portions representing 

 the leaves of each group were cut out and weighed on a balance 

 sensitive to 0.5 milligrams. Knowing the weight of one square deci- 

 meter of the solio paper, the area of the solio leaves was easily cal- 

 culated. The area thus obtained is, of course, that of one side of 

 the leaves only and must be doubled for the total leaf area. To get 

 the total transpiring area it was necessary to add to the leaf areas 

 thus determined the area of the culms. These were cut off at the 

 surface of the wax seal. The area was determined by multiplying 

 the culm length of each plant by its average diameter by 3.1416. 

 Four diameter readings were made for each culm by placing it on a 

 millimeter ruler under a reading glass. 



Finally, the loss of weight in grams of each container for each 

 period was divided by the total transpiring area of the plants it con- 

 tained expressed in square decimeters. The quotient was the trans- 

 ]>iration in grams per sqi;are decimeter for the period imder con- 

 sideration. 



A slight source of error in this method might arise from the fact 

 that final transpiring surfaces were used to calculate earlier water 

 losses. However, it has been pointed out that the first leaves weie 

 apparently full-grown at the beginning of the transpiration meas- 

 urements and any increase in growth would probably be propor- 

 tional in all the plants used. 



Several weighings were made to determine the uniformity in 

 weight of the solio paper. The readings are shown in the following 

 table. 



