Family S. COLLEMACEAE. 



This is without doubt the most clearly defined and natural lichen- 

 family, its representatives being at once recognized by the presence 

 of the alga Nostoc (except in the genus Hydrothyria) . The generic 

 groups doubtless form a continuous series, having no close relation- 

 ship to generic groups of any other family. According to the spore- 

 characters they are perhaps most closely related to Psoroma and Pan- 

 intn'a of the Pannariaceae. 



All of the members of the family are foliose, from the lowest to 

 the highest ; the algae are numerous and usually quite uniformly 

 distributed throughout the entire thickness of the thallus. For this 



O 



reason they were formerly designated as homoimerous lichens. The 

 highly gelatinous condition of the thallus is due to the gelatinous 

 covering of the algae and not to special gelatinization of the hyphal 

 cell-walls. A Cortical layer is present in the forms above Collema, 

 but none exists in that genus and it should therefore be compared 

 with crustaceous lichens, at least when considered from the stand- 

 point of comparative morphology. The principal reason why it 

 differs essentially from the usual crustaceous lichens is due to the 

 presence of the algal gelatine which gives the thallus a smooth ap- 

 pearance and does not permit the formation of areoles. In some 

 Collemas the thallus-lobes are very small and irregular in form. In 

 all Collemaceae the thallus is considerably folded and rugose, prin- 

 cipally due to inequalities in growth and to the interrelation of 

 hyphae and algae. An area of the thallus bearing comparatively 

 more algae than another area of the same thallus will expand more 

 upon the absorption of moisture and cause the two areas to become 

 disturbed relative to each other due to the force exerted. Subse- 

 quent processes of growth cause these inequalities to become fixed. 

 (Retardation and acceleration of growth due to pressure.) 



In general it may be stated that the morphology of the family in- 

 dicates a comparatively low stage of development, but their physio- 

 logical adaptation is highly developed if we consider them from 

 the standpoint of their origin. The algal symbiont, which was ori- 

 ginally a form of Nostoc closely related to N. commune, has under- 

 gone great changes by way of adapting itself to new environments \ 

 from being dependent upon a constant high degree of moisture it be- 

 came enabled to exist and thrive upon tree-trunks, rocks, etc. ; it 

 also became much reduced in thickness, forming a thin, evenly 



