As in Hymenomycetes, the spores of Hymenolichens are exogenous, and are borne at the tips of basidia which in these lichens are produced on the under surface of the thallus. In Cora the fertile filaments may form a continuous series of basidia over the surface, but generally they grow out in separate though crowded tufts. As these tufts broaden outwards, they tend to unite at the free edges, and may finally present a continuous hymenial layer. Each basidium bears four sterigmata and spores ([Fig. 87] e); paraphyses exactly similar to the basidia are abundant in the hymenium. In Dictyonema the hymenium is less regular, but otherwise it resembles that of Cora. No hymenium has as yet been observed in Corella; it includes, so far as known, one species, C. brasiliensis, which spreads over soil or rocks.
CHAPTER IV
REPRODUCTION
1. REPRODUCTION BY ASCOSPORES
A. Historical Survey
The earliest observations as to the propagation of lichens were made by Malpighi[525] who recorded the presence of soredia on the lichen plant and noted their function as reproductive bodies. He was followed after a considerable interval by Tournefort[526] who placed lichens in a class apart owing to the form of the fruit: “This fruit,” he writes, “is a species of bason or cup which seems to take the place of seeds in these kinds of plants.” He figures Ramalina fraxinea and Physcia ciliaris, both well fruited specimens, and he represents the “minute dust” contained in the fruits as subrotund in form. The spores of Physcia ciliaris are of a large size and dark in colour and were undoubtedly seen by Tournefort. Morison[527], in his History of Oxford Plants, published very shortly after, dismissed Tournefort’s “seeds” as being too minute to be of any practical interest.
Micheli[528], with truer scientific insight, made the fruiting organs the subject of special study. He decided that the apothecia were floral receptacles, receptacula florum, and that the spores were the “flowers” of the lichen. He has figured them in a vertical series in situ, in a section of the disc of Solorina saccata[529] and also in a species of Pertusaria[529], in both of which plants the ascospores are unusually large. He adds that he had not so far seen the “semina.”
Micheli’s views were not shared by his immediate successors. Dillenius[530] scarcely believed that the spores could be “flowers” and, in any case, he concluded that they were too minute to be of any real significance in the life of the plant.
Linnaeus[531], and after him Necker[532], Scopoli[533] and others describe the apothecia as the male, the soredia as the female organs of lichens. These old time botanists worked with very low powers of magnification, and easily went astray in the interpretation of imperfectly seen phenomena.