Sampson and Allen : physical factors on transpiration. 37 



A comparison of the tables indicates that the young Helianthus 

 with cotyledons lost more by transpiration than did the older leaves. 

 On the contrary, the number of stomata on the upper side of the 

 cotyledons are fewer per square millimeter than they are on the 

 foliage leaves. The loose spongy tissue and poorly developed cu- 

 ticular layer found on the cotyledons probably accounts for the 

 greater loss of water. 



Only slight variations occur in each series, the greatest being 

 about 5 mm. The loss at night seems to be more uniform than in 

 the daytime. 



In selecting plants in the alpine formations to be used in potome- 

 ters, great care was taken to get them as uniform in size as possi- 

 ble, and in a good healthy condition. Three plants of the same 

 species were tested and two checks were made. 



Potometers were set up August 17, and measurements closed 

 August 20, 1906. 



TABLE II. 

 Table of Physical Factors. 



62 



473 



187 



71 



16.8X2.8 

 5.6X.8 

 S.1X.7 



