COLLEMA. 



277 



A comparatively small, but peculiar family, distinguished 

 from all other Lichens by their vegetative system. The 

 fructification resembles chiefly that of the Gymnocarpi ; but, 

 in some species, it appears to be truly angiocarpous. Hence 

 the genus Collema may be appropriately considered inter- 

 mediately between the Gymnocarpi and Angiocarpi. While 

 the reproductive system is distinctly lichenoid, the vegetative 

 system is somewhat algoid : on this account the Collemas have 

 been regarded by many authors as abnormal Algse, growing 

 in the air, or as aquatic, algoid Lichens. Their vegetative 

 system does not exhibit the ordinary divisions into three 

 distinct layers or tissues, which are here generally confused 

 and modified. What corresponds to the medullary tissue 

 of other Lichens consists of a loose network of interlacing 

 moniliform or beaded filaments, — formed by the apposition 

 in linear series of minute globular cells, — and of delicate, 

 narrow, branching tubes. In many species this tissue con- 

 stitutes the bulk of the thallus. There is no distinct gonidic 

 layer ; the gonidia are represented either by green globules, 

 frequently arranged in rows, but not united to each other, and 

 interspersed among the meshes of the filamentous tissue, — 

 or in some species, by chlorophyll dissolved in the mucilage 

 of the thallus. In a few there is a cortical layer composed of 



