HEAT PRODUCED BY LEAVES 279 



the weight used being 70 grams. Another was lightly filled 

 with just half the amount. As will be noted (table 1) this had 

 no effect on the temperatures. 



According to Molisch, the rises in temperature can not be 

 attributed to the respiration and work of microorganisms, since 

 he was, in several cases, careful to examine the leaves immedi- 

 ately after the first maximum, and, though finding the leaves 

 brown and dead, they were apparently free from microorgan- 

 isms. However, after four days (the time of the second maxi- 

 mum) he found the leaves overgrown with microorganisms, 

 chiefly fungi. In only one case in our experiments were any 

 fungal growths evident, (and these but slight) and in very few 

 was there any evidence of bacterial growth. 



As will be seen from the tables, it was impossible in any. way 

 to duplicate the results obtained by Molisch. In only two ex- 

 periments was there any appreciable rise in temperature. The 

 first, in leaves of Tilia, (table 5) which rose 6.4° in seven hours, 

 is easily explained when the first reading of the Dewar flask is 

 noted. The leaves were picked in the sunshine on a hot day, 

 and immediately placed in the flasks. The temperatures, as 

 is observed, differ by only one degree (due to the much greater 

 mass in the carboy) which proves that the heat exhibited by the 

 leaves in the carboy was simply a result of the previous external 

 temperature. 



The leaves exhibiting the other rise in temperature were Robi- 

 nia. As will be noticed (table 6, B), the rise did not come for 

 six days (after which length of time Molisch found plentiful 

 growth of microorganisms) and, as stated in the table, the con- 

 dition of the leaves gave strong evidence of bacterial decay. 



Molisch used leaves which were fully mature, their period of 

 growth past, while the leaves first used in our experiments were 

 very young, growing in the open in the early spring. Since in 

 young leaves respiration is often more intense than in fully grown 

 leaves, one would suppose that more positive results would be 

 obtained in the spring than in the fall. However, in order that 

 all the conditions under which Molisch worked would be as 

 nearly duplicated as possible, experiments were made in the 



