LICHEN ALGAE 63 



algal cells had been also noted by Krabbe^ in the young podetia of some 

 species of Cladonia, the change in form being due to the continued pressure 

 in one direction of the parallel hyphae. 



Isolated algal cells have been observed within the cortex of various 

 lichens. They are carried thither by the hyphae from the gonidial zone in 

 the process of cortical formation, but they soon die off as in that position 

 they are deprived of a sufficiency of air and of moisture. Forssell^ found 

 Xanthocapsa cells embedded in the hymenium of Omphalaria Heppii. They 

 were similar to those of the thallus, but they were not associated with hyphae 

 and had undergone less change than the thalline algae. 



C. Constancy of Algal Constituents 



Lichen hyphae of one family or genus, as a rule, combine with the same 

 species of alga, and the continuity of genera and species is maintained. 

 There are, however, related lichens that differ chiefly or only in the characters 

 of the gonidia. Among such closely allied genera or sections of genera may 

 be cited Sticta with bright-green algae and the section Stictina with blue- 

 green; Peltidea similarly related to Peltigera and Nephroma to N ephromium. 

 In the genus Solorina, some of the species possess bright-green, others blue- 

 green algae, while in one, .S". crocea^, there is an upper layer of small bright- 

 green gonidia that project in irregular pyramids into the upper cortex ; 

 while below these there stretches a more or less interrupted band of blue- 

 green Nostoc cells. The two layers are usually separated by strands of 

 hyphae, but occasionally they come into close contact, and the hyphal 

 filaments pass from one zone to the other. In this genus cephalodia con- 

 taining blue-green Nostoc are characteristic of all the "bright-green" species. 

 Harmand* has recorded the presence of two different types of gonidia in 

 Lecanora atra f subgrumosa; one of them, the normal Protococcus alga of the 

 species, the other, pale-blue- green cells oi Nostoc affinity. 



Forssell'* states that in Lecanora (Psoroma) hypnorum, the normal bright- 

 green gonidia of some of the squamules may be replaced by Nostoc. In that 

 case they are regarded as cephalodia, though in structure they exactly 

 resemble the squamules of Pafinaria pezizoides, and Forssell considers that 

 there is sufficient evidence of the identity of the hyphal constituent in these 

 two lichens, the alga alone being different. 



It may be that in Archilichens with a marked capacity to form a second 

 symbiotic union with blue-green algae, a tendency to revert to a primitive 

 condition is evident — a condition which has persisted wholly in Peltigera 

 with its Nostoc zone, but is manifested only by cephalodia formation in the 



' Krabbe 1891. ^ Forssell 1885. ' Hue 1910. * Harmand 1913, p. 1050. 



5 Forssell 1886. 



