[Illustration: Fig. 87. _Cora Pavonia_ Fr. Vertical section of thallus. _a_, upper cortex; _b_, gonidial layer; _c_, medulla and lower cortex of crenate cells; _d_, tuft of fertile hyphae. × 160. _e_, basidia and spores × 1000 (after Johow).] In _Corella_, the mature lichen is squamulose or consists of small lobes; in _Dictyonema_ there is a rather flat dimidiate expansion; in both the alga is _Scytonema_, the trichomes of which largely retain their form and are surrounded by parallel growths of branching hyphae. The whole tissue is loose and spongy. _Corella_ spreads over soil on a white hypothallus without rhizinae. In the other two genera which live on soil, or more frequently on trees, there is a rather extensive formation of hold-fast tissue. When the dimidiate thallus grows on a rough bark, rhizoidal strands of hyphae travel over it and penetrate between the cracks; if the bark is smooth, there is a more continuous weft of hyphae. In both cases a spongy cushion of filamentous tissue develops at the base of the lichen between the tree and the bracket thallus. There is also in both genera an encrusting form which Johow regarded as representing a distinct genus _Laudatea_, but which Möller found to be merely a growth stage. Möller[524] judged from that and from other characteristics that the same fungus enters into the composition of both _Cora_ and _Dictyonema_ and that only the algal constituents are different. C. SPORIFEROUS TISSUES 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