192 



spreading layer upon the substratum ; its greatest adaptations are 

 perhaps in relation to new habitats. Although the Collemaceae may 

 perhaps not be able to resist such extremes of dryness and tempera- 

 ture as most lichens, yet from the original habits of the predomina- 

 ting symbiont (JVostoc) it is evident that the adaptive changes were 

 even greater than in the majority of lichens. The changes are, of 

 course, primarily due to the interaction of both symbionts since the 

 establishment of the lichen phylogeny. 



The fungal symbiont is doubtless derived from some group of the 

 Patellariaceae. Whether the family has been developed from one or 

 from several fungal ancestors is questionable. From the study of the 

 genera it seems that two ancestors are, perhaps, represented, one for 

 Collema, JLeptogium and Mallotium, and one for Hydrothyria. In 

 this family, also, the spores are retrogressive in their development, as 

 is indicated by the absence of coloration and a tendency toward sim- 

 pler forms, which is shown by the existence of non-septate and thin- 

 walled spores. Soredia are comparatively few, due, no doubt, to the 

 difficulty of enclosing the algal gelatine by the hyphae. Haustoria 

 proper can not be said to exist in the family. The hyphae lie in con- 

 tact with the algae and pass through the gelatinous substance, which 

 position is sufficient to enable the hyphae to absorb the assimilated 

 food-substances of the algae. 



As to the algal symbiont, it presents the general appearance of N. 

 commune; the threads are of about the same length ; the heterocysts 

 are, perhaps, somewhat fewer in number ; several observers have 

 cultivated it in various media, free from its fungal symbiont. The al- 

 gal symbiont of Hydrothyria is Rivularia. 



As already stated, the members of the Collemaceae are foliose ; 

 the color of the thallus and apothecia varies from greyish blue to 

 dark blue. They are attached to the substratum by the gelatine and 

 rhizoids, also by a central attachment, (umbilicus) as in Umbilicaria, 

 and are quite common throughout the territory. 



Key to the Genera. 

 Cortical layers wanting. 1. Collema. 



Cortical layers present. 

 Venation wanting. 



Long rhizoids not present. 2. Leftogium. 



Long rhizoids present. 3. Mallotium. 



Venation on lower surface of thallus 4. Hydrothyria. 



