14 



Halsted : Mycological Notes 



trees the fact of the relation of sun exposure to the development 

 of the bhght became known. . . - 



The first theory employed was the position of the leaf upon 

 the twig, but there were so many exceptions to the rule that the 

 latter was not established. Tlic cherry leaf has a way of bending 

 the two halves of the blade upward so that the underside of the 



^ 



leaf is the only one in sight and may be unequally exposed accord- 

 ingly. Such leaves upon any tree hang in all possible positions 

 in relation to the sun, but taking a tree as a whole, there is a 

 shady and a sunny side. In like manner there arc some of the 

 leaves so posed that one of the upturned halves is southward. 

 When such leaves are situated upon the south side of the tree the 

 conditions are fulfilled for the production of an instance when the 

 fungus of the leaf is largely confined to the sunny half 



V 



Fig. I 



The observations were made in part upon small trees and the 

 observer's eye could take the position of sun, that is, the rays of 

 sunlight at mid-day were parallel to the line of vision when the 



