tqiq] 



WEAVER &- MOGENSEN— TRANSPIRATION 



401 



(fig. 2). The areas were determined by removing the leaves from 

 the plants, measuring the length of the leaf fascicle and the average 

 diameter of the fiat faces, as well as the diameter of the cylinder 

 formed by the two or three leaves in the fascicle when the flat faces 

 were appressed against each other. From these data the actual 

 surface area was calculated. Practically no needles were shed 

 during the winter. 



Fig. 3 shows the total average daily losses of six yellow pines 

 from September 24 to January i. An examination of these graphs 

 reveals a striking similarity. The highest losses are from tree 



Fig. 3. — Average daily losses in grams from six 3-year-old yellow pines from 

 September 24 to January i ; heavy line represents mean temperature for the several 

 periods. 



no. I with the greatest leaf area (6.428 sq. dm.), while the lowest 

 losses are plotted from data obtained from tree no. 2, which had 

 an area of only 4.1 sq. dm. Comparative losses per unit area are 

 given elsewhere. Data from the other two pines were omitted 

 in this figure for the sake of clearness. A general relation between 

 temperature and transpiration was clearly evident. The relation 

 to humidity was not so apparent. 



The entire period from September 24 to October 16 is character- 

 ized by relatively high transpiration losses, after which there is a 

 decided falling off. On October 1 1 the stomata were found to be 

 closed. The midwinter transpiration losses are exceedingly small. 

 Weighings made on February 7 and after a period of prolonged cold 



