IOO 



Evernia, etc.). It is a single-celled, spherical alga, and its only 

 mode of reproduction while in the symbiotic association is by direct 

 division ; it has been successfully cultivated in culture media, inde- 

 pendent of its fungal associate, and differs from the free form in 

 being considerably larger. It varies, however, in size in different 

 lichen-species ; the nucleus is usually quite distinct in fresh material ; 

 the cell-wall is about normal in thickness and sometimes penetrated 

 by the haustoria. In old herbarium material the algae become pale- 

 green. As far as I have been able to observe, this alga is never as- 

 sociated with any other symbiotic alga within the thallus. 



2. Chroolepus {Trentofohlia) umbrina. — This alga occurs quite 

 uniformly in the Graphidaceae and the lower Verrucariaceae. It con- 

 sists of sparingly branching filaments of loosely united cells ; the 

 individual cells are irregular in form, bright green, and contain irreg- 

 ular oleagenous granules of a reddish brown color, whose origin and 

 function is unknown. In the free state these algae occur quite com- 

 monly on bark, rocks, soil, etc., in places somewhat shaded and 

 moist. New cells are formed by division, in the majority of cases, 

 at right angles to the filament ; less frequently longitudinally. 



The cells of the algae are enclosed by haustoria, but in no case 

 have I been able to observe the penetration of the algal wall. 



3. Pleurococcus vulgaris Menegh. — This species occurs in Der- 

 matocarfon, Acarospora and in Endocarpon. It is single-celled, 

 more or less irregular in outline and much larger than the free form, 

 bright green with a bluish tinge while in the fresh state, and is quite 

 common upon bark, rock, old fences, etc. 



4. Dactylococcus infusionum. — The cells of this alga are elliptical 

 and rather small ; they usually are united in colonies of eight or 

 twelve, sometimes fewer, and occur in Nephroma, Solorina and 

 Psoroma. 



5. Nostoc commune {Nostoc lichenoides). — This alga occurs in 

 the family Collemaceae (excepting Hydrothyrid). Whether it is 

 Nostoc commune or not may be questioned. It is in all essential char- 

 acters like the Nostoc occurring in the root-tubercles of Cycas revo- 

 luta and an ordinary Nostoc of our ponds and fresh-water lakes ; 

 the chains are always imbedded in a thick layer of gelatine or 

 mucilage, through which the hyphae of the thallus ramify without 

 forming haustoria about the algal cells. In size and development it 

 differs but little from the free form of Nostoc commune, in the 



