_c._ SHADE LICHENS. Many species grow where the light is abundant though diffuse. Those on tree-trunks rarely receive direct illumination and may be generally included among shade-plants. Wiesner found that corticolous forms of _Parmelia saxatilis_ grew best with an illumination between 1/8 and 1/17 of full light, and _Pertusaria amara_ from 1/12 to 1/21; both of them could thrive from 1/3 to 1/56, but were never observed on trees in direct light. _Physcia ciliaris_, which inhabits the trunks of old trees, is also a plant that prefers diffuse light. In warm tropical regions, lichens are mostly shade-plants: Wiesner records an instance of a species found on the aerial roots of a tree with an illumination of only 1/250. In a study of subterranean plants, Maheu[877] takes note of the lichens that he found growing in limestone caves, in hollows and clefts of the rocks, etc. A fair number grew well just within the opening of the caves; but species such as _Cl. cervicornis_, _Placodium murorum_ and _Xanthoria parietina_ ceased abruptly where the solar rays failed. Only a few individuals of one or two species were found to remain normal in semi-darkness: _Opegrapha hapalea_ and _Verrucaria muralis_ were found at the bottom of a cave with the thallus only slightly reduced. The nature of the substratum in these cases must however also be taken into account, as well as the light influences: limestone for instance is a more favourable habitat than gypsum; the latter, being more readily soluble, provides a less permanent support. Maheu has recorded observations on growth in its relation to light in the case of a number of lichens growing in caves. _Physcia obscura_ grew in almost total darkness; _Placodium murorum_ within the cave had lost nearly all colour; _Placodium variabile_ var. deep within the cave, sterile; _Opegrapha endoleuca_ in partial obscurity; _Verrucaria rupestris_ f. in total obscurity, the thallus much reduced and sterile; _Verrucaria rupestris_ in partial obscurity, the asci empty; _Homodium_ (_Collema_) _granuliferum_ in the inmost recess of the cave, sterile, and the hyphae more spongy than in the open. Siliceous rocks in darkness were still more barren, but a few odd lichens were collected from sandstone in various caves: _Cladonia squamosa_, _Parmelia perlata_ var. _ciliata_, _Diploschistes scruposus_, _Lecidea grisella_, _Collema nigrescens_ and _Leptogium lacerum_.