IOI 



majority of instances the individual cells being, perhaps, a little lar- 

 ger. The heterocysts, though well developed, are comparatively 

 few in number. This alga has also been cultivated in culture-me- 

 dia independently of the fungal symbiont. 



6. Rivularia nitida. This alga occurs in Polychidium^ Ompha- 

 laria, Lichina, etc. Like JVostoc it occurs in chains enclosed by a 

 gelatinous layer through which the hyphae pass ; its cells are quite 

 irregular in outline, much more so than in the free form ; heterocysts 

 are wanting. 



7. Poly coccus punctiformis. This alga occurs in Peltigera, 

 Pannaria, and Stictina. The cells are elliptical and imbedded in a 

 gelatinous substance, sometimes in colonies of rfour or more. They 

 are somewhat larger than the free forms which occur quite 

 frequently upon trees, fences and upon lichen-thalli. 



8. Gloeocapsa polydermatica. I have found this alga quite con- 

 stantly in Baeomyces roscus and in Omphalaria umbella as already 

 indicated. The individual cells are spherical, with a thick gelatinous 

 layer, and occur quite frequently in colonies of four or more. I 

 have been unable to detect any difference between this and the free 

 form. It is questionable whether it occurs constantly in Baeomyces. 

 In Omphalaria it is associated with Nostoc. 



9. Sirosiphon pulvinatns. This alga, represented in Ephebe 

 pubescens, is very much like the normal form in its growth and mode 

 of branching. It grows from an apical cell which cuts off new cells 

 in both planes forming branches of two or more cell-rows. This spe- 

 cies also enters into the formation of the cephalodia of Stereocaulon. 



For further detailed description and comparative study of the 

 lichen-algae, the student is referred to any standard work treating of 

 fresh-water algae. In making such comparisons, it is well to remem- 

 ber that the algae of lichens are, as a rule, larger and more irregular 

 in form than the corresponding free algae. In the majority of lichens 

 the algal species can not be absolutely determined. It is perfectly 

 natural that the algae, as well as the fungal portion, should have un- 

 dergone considerable structural as well as functional changes since 

 their symbiotic association. 



Plate 5 will aid in showing the relation of lichens to algae and 

 fungi, that is their polyphyletic origin, but no attempt is made to in- 

 dicate the polyphyletic origin of the fungal groups. The diagram 

 also shows the relative position of the lichen-families in the scale of 

 development. 



