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[299] 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 Nephromium. In the genus Solorina, some of the species possess bright-green, others blue-green algae, while in one, S. crocea[300], 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 containing blue-green Nostoc are characteristic of all the “bright-green” species. Harmand[301] 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 of Nostoc affinity.
Forssell[302] 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 Pannaria 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 Peltidea section of the genus. In this connection, however, we must bear in mind Forssell’s view that it is the Archilichens that are the more primitive[303].
The alien blue-green algae with their gelatinous sheaths are adapted to the absorption and retention of moisture, and, in this way, they doubtless render important service to the lichens that harbour them in cephalodia.
D. Displacement of Algae within the Thallus
a. Normal displacement. Lindau[304] has contrasted the advancing apical growth of the creeping alga Trentepohlia with the stationary condition of the unicellular species that multiply by repeated division or by sporulation, and thus form more or less dense zones and groups of gonidia in most lichens. The fungus in the latter case pushes its way among the algae and breaks up the compact masses by a shoving movement, thus letting in light and air. The growing hypha usually applies itself closely round an algal cell, and secondary branches arise which in time encircle it in a network of short cells. In the thallus of Variolaria[305] the hyphae from the lower tissues, termed push-hyphae by Nienburg[306], push their way into the algal groups and filaments composed of short cells come to lie closely round the individual gonidia. Continued growth is centrifugal, and the algae are carried outward with the extension of the hyphae ([Fig. 12]). Cell-division is more active at the periphery, that being the area of vigorous growth, and the algal cells are, in consequence, generally smaller in that region than those further back, the latter having entered more or less into a resting condition, or, as is more probable, these smaller cells are aplanospores not fully mature.
b. Local displacement. Specimens of Parmelia physodes were found several times by Bitter, the grey-green surface of which was marbled with whitish lines, caused by the absence of gonidia under these lighter-coloured areas. The thallus was otherwise healthy as was manifested by the freely fruiting condition: no explanation of the phenomenon was forthcoming. Bitter compared the condition with the appearance of lighter areas on the thallus of Parmelia obscurata.
Something of the same nature was observed on the thallus of a Peltigera collected by F. T. Brooks near Cambridge. The marking took the form of a series of concentric circles, starting from several centres. The darker lines were found on examination to contain the normal blue-green algal zone, while the colour had faded from the lighter parts. The cause of the difference in colouration was not apparent.