VI. HYMENOLICHENS
A. Supposed Affinity with other Plants
Lichens in which the fungal elements belong to the Hymenomycetes are confined to three tropical genera. They are associated with blue-green algae and are most nearly related to the Thelephoraceae among fungi. The spores are borne, as in that family, on basidia.
Fig. 86. Cora Pavonia Fr. (after Mattirolo).
The best known Hymenolichen, Cora Pavonia ([Fig. 86]), was discovered by Swartz[516] during his travels in the W. Indies (1785-87) growing on trees in the mountains of Jamaica, and the new plant was recorded by him as Ulva montana. Gmelin[517] also included it in Ulva in close association with Ulva (Padina) Pavonia, but that classification was shortly after disputed by Woodward[518] who thought its affinity was more nearly with the fungi and suggested that it should be made the type of a new genus near to Boletus (Polystictus) versicolor. Fries[519] in due time made the new genus Cora, though he included it among algae; finally Nylander[520] established the lichenoid character of the thallus and transferred it to the Lecanorei.
It was made the subject of more exact investigation by Mattirolo[521] who recognized its affinity with Thelephora, a genus of Hymenomycetes. Later Johow[522] went to the West Indies and studied the Hymenolichens in their native home. The genera and species described by Johow have been reduced to Cora and Dictyonema; a new genus Corella has since been added by Wainio[523].
Johow found that Cora grew on the mountains usually from 1000 to 2000 ft. above sea-level. As it requires for its development a cool damp climate with strong though indirect illumination, it is found neither in sunny situations nor in the depths of dark woods. It grows most freely in diffuse light, on the lower trunks and branches of trees in open situations, but high up on the stem where the vegetation is more dense. It stands out from the tree like a small thin bracket fungus, one specimen placed above another, with a dimidiate growth similar to that of Polystictus versicolor. Both surfaces are marked by concentric zones which give it an appearance somewhat like Padina Pavonia. These zones indicate unequal intercalary growth both above and below. The whole plant is blue-green when wet, greyish-white when dry, and of a thin membranaceous consistency.
B. Structure of Thallus
There is no proper cortex in any of the genera, but in Cora there is a fastigiate branching of the hyphae in parallel lines towards the upper surface; just at the outside they turn and lie in a horizontal direction, and, as the branching becomes more profuse, a rather compact cover is formed. The gonidia, which consist of blue-green Chroococcus cells, lie at the base of the upward branches and they are surrounded with thin-walled short-celled hyphae closely interwoven into a kind of cellular tissue. The medulla of loose hyphae passes over to the lower cortex, also of more or less loose filaments. The outermost cells of the latter very frequently grow out into short jagged or crenate processes ([Fig. 87]).
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.