The natural increase of lichen plants may primarily be sought for in the dispersal of the spores produced in the fruiting-bodies. These are ejected, as in fungi, by the pressure of the paraphyses on the mature ascus. The spores are then carried away by wind, water, insects, etc. In a few lichens gonidia are enclosed in the hymenium and are ejected along with the spores, but, in most, the necessary encounter with the alga is as fortuitous, and generally as certain, as the pollination of anemophilous flowers. A case of dispersal in _Sagedia microspora_ has been described by Miyoshi[915] in which entire fruits, small round perithecia, were dislodged and carried away by the wind. The addition of water caused them to swell enormously and brought about the ejection of the spores. Areas covered by the thallus are also being continually enlarged by the spreading growth of the hypothallus. _a._ DISPERSAL OF CRUSTACEOUS LICHENS. These lichens are distributed fairly equally on trees or wood (corticolous) and on rocks (saxicolous). Some species inhabit both substrata. As regards corticolous lichens that live on smooth bark such as hazel or mountain-ash, the vegetative body or thallus is generally embedded beneath the epidermis of the host. Soredia are absent and the thallus is protected from dispersal. In these lichens there is rather an abundant and constant formation of apothecia or perithecia. Other species that affect rugged bark and are more superficial are less dependent on spore production. The thallus is either loosely granular, or is broken up into areolae. The areolae are each a centre of growth, and with an accession of moisture they swell up and exert pressure on each other. Parts of the thallus thus become loosened and are dislodged and carried away. If anchored on a suitable substratum they grow again to a complete lichen plant. Sorediate lichens are dependent almost wholly on these bud-like portions for increase in number; soredia are easily separated from the parent plant, and easily scattered. Darbishire[916] noted frequently that small _Poduridae_ in moving over the surface of _Pertusaria amara_ became powdered with soredia and very evidently took a considerable part in the dissemination of the species.