190 Trans. Acad. Sci. of St. Louis. 



dichotomously and by adventive branches, the latter standing 

 usually at an angle of about 80°. Examined more minutely 

 the thallus is found to consist of three layers, an external 

 cortical layer, in which the hyphae are intricately entangled 

 into a network of greater or less density, a middle layer with 

 the gonidia, very much looser in texture, and a central or 

 medullary layer in which the hyphae are parallel to the long 

 axis of the thallus. Towards the tips both the main trunk 

 and the branches rapidly decrease in diameter. As the thal- 

 lus grows older, the cortical layer weakens and breaks in 

 manv places, exposing the medullary layer, with rings of 

 cortex about it. This is specially marked when the thallus is 

 wet and stretched. On the long hanging forms of Usnea^ 

 apothecia are very rarely found. 



On Long Island and in Connecticut Usnea barhata (L.) var. 

 plicata, Fr.,* is very common particularly on Juniperus Vir- 

 giniana and Pinus rigida^ while farther north in Massachu- 

 setts and Maine I have found U. harhata (L.) var. dasypoga^ 

 Fr., the more prevalent form, growing on the white spruce, 

 Ficea alba, and the balsam fir, Abies balsamea. This apparent 

 predilection for the short-leaved Coniferae is probably due to 

 the fact that the long threads are easily caught and held by 

 branches with short leaves extending like brushes in all direc- 

 tions. Deciduous-leaved trees and the longer-leaved pines do 

 not present such favorable supports; pieces of the lichen 

 placed on such trees rarely become fixed, while those placed 

 on Juniperus and Abies remained fast as long as they were 

 kept under observation. 



If one examines a tree covered by the lichen, one will find 

 pieces varying from an inch in length to great masses on one 

 and the same branch. The threads are rarely straight, but 

 are bent and twisted among themselves, and wind in be- 

 tween and around the leaves and stem in a tangle which it is 

 almost impossible to unravel. (See PI. XVI.) This cannot 

 be the result of growth, as the lichen grows so very slowly, 

 and such tangles I have seen form in a few weeks. Nor is 



* I am indebted to Miss Clara E. Cummings for the determination of 

 these varieties. 



