Schwendener[185] meanwhile was engaged on his study of lichen anatomy. Though at first he adhered to the then accepted view of the genetic connection between hyphae and gonidia, his continued examination of the vegetative development led him to publish a short paper[186] in which he announced his opinion that the various blue-green and green gonidia were really algae and that the complete lichen in all cases represented a fungus living parasitically on an alga: in Ephebe, for example, the alga was a form of Stigonema, in the Collemaceae it was a species of Nostoc. In those lichens enclosing bright green cells, the gonidia were identical with Cystococcus humicola, while in Graphideae the brightly coloured filamentous cells were those of Chroolepus (Trentepohlia). This statement he repeated in an appendix to the larger work on lichens[187] and again in the following year[188] when he described more fully the different gonidial algae and the changes produced in their structure and habit by the action of the parasite: “though eventually the alga is destroyed,” he writes, “it is at first excited to more vigorous growth by contact with the fungus, and in the course of generations may become changed beyond recognition both in size and form.” In support of his theory of the composite constitution of the thallus, Schwendener pointed out the wide distribution and frequent occurrence in nature of the algae that become transformed to lichen gonidia. He claimed as further proof of the presence of two distinct organisms that, while the colourless filaments react in the same way as fungi on the application of iodine, the gonidia take the stain of algal membranes.
G. Synthetic Cultures
Schwendener’s “dual hypothesis,” as it was termed, excited great interest and no little controversy, the reasons for and against being debated with considerable heat. Rees[189] was the first who attempted to put the matter to the proof by making synthetic cultures. For this purpose he took spores from the apothecium of a Collema and sowed them on pure cultures of Nostoc, and as a result obtained the formation of a lichen thallus, though he did not succeed in producing any fructification. He observed further that the hyphal filaments from the germinating spore died off when no Nostoc was forthcoming.
Bornet[190] followed with his record of successful cultures. He selected for experiment the spores of Physcia (Xanthoria) parietina and was able to show that hyphae produced from the germinating spore adhered to the free-growing cells of Protococcus[191] viridis and formed the early stages of a lichen thallus. Woronin[192] contributed his observations on the gonidia of Parmelia (Physcia) pulverulenta which he isolated from the thallus and cultivated in pure water. He confirmed the occurrence of cell division in the gonidia and also the formation of zoospores, these again forming new colonies of algae identical in all respects with the thalline gonidia. He was able to see the germinating tube from a lichen spore attach itself to a gonidium, though he failed in his attempts to induce further growth. In our own country Archer[193] welcomed the new views on lichens, and attempted cultures but with very little success. Further synthetic cultures were made by Bornet[194], Treub[195] and Borzi[196] with a series of lichen spores. They also were able to observe the first stages of the thallus. Borzi observed spores of Physcia (Xanthoria) parietina scattered among Protococcus cells on the branch of a tree. The spores had germinated and the first branching hyphae had already begun to encircle the algae.
Fig. 5. Endocarpon pusillum Hedw. Asci and spores, with hymenial gonidia × 320 (after Stahl).
Fig. 6. Endocarpon pusillum Hedw. Spore germinating in contact with hymenial gonidia × 320 (after Stahl).
Additional evidence in favour of the theory of the independent origin of the colourless filaments and the green cells was furnished by Stahl’s[197] research on hymenial gonidia in Endocarpon ([Fig. 5]). By making synthetic cultures of the mature spores with these bodies, he was able to observe not only the germination of the spores and the attachment of the filaments to the gonidia ([Fig. 6]), but also the gradual building up of a complete lichen thallus to the formation of perithecia and spores.
