Cannon: A method of measuring transpiration 529 



in rate of this plant was 8.03 milligrams per minute for 100 sq. 

 cm. of transpiring surface. The least rate was that of March 

 when 0.22 milligrams for the same area and period was observed, 

 and the highest was in August when the rate was 8.25 milligrams. 



8. The apparatus is so sensitive to changes in the humidity 

 to give an illustration of the delicacy of the method — that it was 



possible to demonstrate and to estimate the transpiration of Echino- 

 cactus during the extremely hot and dry climatic conditions of 

 June, at a time when the transpiration of all desert plants was very 

 feeble. It is of interest to observe that the cobalt nitrate method 

 as employed to demonstrate transpiration was quite ineffective as 

 a means of showing any transpiration of the plant. 



9. Finally the convenience with which the apparatus may be 

 transported may be mentioned. This feature makes it possible to 

 extend the work into out-of-the-way places, as for example, to 

 study the effects of altitude on the rate of transpiration when the 

 experiments have to be carried on in the mountains. As a single 

 Instance of this use of the method the following may be given : 

 Some experiments on Helenium Hoopesti were performed at differ- 

 ent altitudes in the Catalina Mountains, which are not far distant 

 from the laboratory but which are rough and rather difficult to 

 enter. A portion of the experiments were conducted at an altitude 

 of 7,600 feet and a portion at an altitude of 9,150 feet. The 

 general result of these studies may be summed up by the state- 

 ment that they tended to support the view that the altitude directly 

 affects the rate of transpiration, which in the present case was 

 greater'at the higher altitude. 



Desert Botanical Laboratory of the 

 Carnegie Institution, 



