i 9 4 



locular ; the wall and septa are quite thin and translucent or trans- 

 parent. In general the spores present degenerative characters,, 

 indicated by the thin wall, absence of color, and the apparent ten- 

 dency toward simple nonseptate forms. Frequently the septa be- 

 come indistinct. In the retrogressive transition-forms from multi- 

 locular to transversely septate we find septa formed diagonally to the 

 normal direction (longitudinal and transverse), which are very in- 

 constant in their occurrence. 



As already stated elsewhere, the alga of Collema has been culti- 

 vated in artificial media. It would be interesting to demonstrate 

 whether or not the Nostoc of any Collema is capable of existing 

 without its fungal symbiont and of leading the life of an indepen- 

 dent Nostoc without the aid of culture media ; also, whether the fun- 

 gal symbiont is capable of leading an independent existence. 



Collemas are propagated vegetatively either by means of soredia 

 or from portions of the thallus. The thallus is also endowed with 

 continuous marginal growth. Each lobe is capable of forming the 

 center of a new individual ; it may or may not sever its connection 

 with the mother plant. 



The species occur upon bark, mosses, rarely upon rocks, on soil, 

 seldom partially or wholly submerged in water ; they are plentiful in 

 the northern and north-temperate zones, both east and west. 



PLATE 61. 

 Collema pulposum Ach. 

 i. Thallus natural size. 



2. Lobe of thallus with apothecia, magnified. 



3. Section of apothecium. 



4. Section of thallus. 



5. Paraphysis and spore-sac. 



6. Spores. 



7. Algae. 



2. Leptogium Ach. ; S. F. Gray, Brit. PI. 1 : 400. 1821. 

 In all its essential characters this genus closely resembles Collema,. 

 from which it is no doubt phylogenetically derived. As compared 

 with Collema, the thallus-lobes are larger and somewhat more rigid ; 

 the rhizoids are comparatively few and slender. The essential dif- 

 ference becomes apparent upon the examination of vertical sections y 



