124 



3. Longitudinal radial section of thallus. 



a, thalloid wart ; b, thin outer layer of hyphal network ; c, inner me- 

 chanical tissue. 



4. Spore-sac with spores and paraphysis. 



5. Spores. 



6. Algae. 



3. Stereocaulon Schreb. Gen. PI. 768. 1791- 

 The question whether this genus is phylogenetically derived from 

 Pilophoron must for the time being remain undecided. It certainly 

 indicates a close relationship, though considerably higher in the scale 

 of development. 



The primary thallus is practically wanting and we will therefore, 

 limit our discussion to the podetium (secondary thallus) ; this is a 

 typically fruticose structure, much branched, the branches being 

 cylindrical, more or less twisted and otherwise contorted, thus form- 

 ing quite a contrast to the podetium of the foregoing genus. The 

 entire outer surface is covered by hyphal branches which in this 

 genus form warty or branching isidioid outgrowths in association with 

 the symbiotic algae. The alga-bearing tissue closely resembles 

 that of Pilophoron though it is more highly developed structurally. 

 No cortical tissue is, however, present. In regard to the algae the 

 student will encounter difficulties ; it will be found that a number of 

 species are quite constantly present. They are a species resembling 

 CystococcurS humicola, Polycoccus punctiformis, a species of Scyto- 

 nema, Sirosiphon pulvinatus, besides other occasional forms. Natu- 

 rally one is at a loss to know which is the alga peculiar to this group 

 of lichens. After considerable study the conclusion was reached that 

 a bright green alga closely resembling Cystococcus humicola is the 

 form peculiar to Stereocaulon. It occurs by far the most frequently, 

 while the others are instances of contingent symbiosis. The numerous 

 hyphal projections from the outer surface of the podetium form a 

 suitable structure to retain algae and soredia carried by the wind. I 

 find that Polycoccus punctiformis is enclosed by a hyphal tissue foreign 

 to Stereocaulon which is doubtless soredia of Nephromium, Stictina 

 or some other group of lichens in which the symbiotic alga is P. 

 punctiformis. Sirosiphon and Scytonema-vccur much less frequently. 

 The algae which seem normal to Stereocaulon differ from the free 

 Cystococcus in that the cells are quite irregular in form and size ; 

 they also have a tendency to arrange themselves in groups or even 

 in chains ; it may also be possible that they are a form of Chroolepus. 



