_b._ ON GENERAL DEVELOPMENT. Lichens are very strongly influenced by abundance or by lack of moisture. The contour of the large majority of species is concentric, but they become excentric owing to a more vigorous development towards the side of damper exposure, hence the frequent one-sided increase of monophyllous species such as _Umbilicaria pustulata_. Wainio[867] observed that species of _Cladonia_ growing in dry places, and exposed to full sunlight, showed a tendency not to develop scyphi, the dry conditions hindering the full formation of the secondary thallus. As an instance may be cited _Cl. foliacea_, in which the primary thallus is much the most abundantly developed, its favourite habitat being the exposed sandy soil of sea-dunes. Too great moisture is however harmful: Nienburg[868] has recorded his observations on _Sphyridium_ (_Baeomyces rufus_): on clay soil the thallus was pulverulent, while on stones or other dryer substratum it was granular—warted or even somewhat squamulose. _Parmelia physodes_ rarely forms fruits, but when growing in an atmosphere constantly charged with moisture[869], apothecia are more readily developed, and the same observation has been made in connection with other usually barren lichens. It has been suggested that, in these lichens, the abrupt change from moist to dry conditions may have a harmful effect on the developing ascogonium. The perithecia of _Pyrenula nitida_ are smaller on smooth bark[870] such as that of _Corylus_, _Carpinu_s, etc., probably because the even surface does not retain water. IV. ILLUMINATION OF LICHENS A. EFFECT OF LIGHT ON THE THALLUS As fungi possess no chlorophyll, their vegetative body has little or no use for light and often develops in partial or total darkness. In lichens the alga requires more or less direct illumination; the lichen fungus, therefore, in response to that requirement has come out into the open: it is an adaptation to the symbiotic life, though some lichens, such as those immersed in the substratum, grow with very little light. Like other plants they are sensitive to changes of illumination: some species are shade plants, while others are as truly sun plants, and others again are able to adapt themselves to varying degrees of light. Wiesner[871] made a series of exact observations on what he has termed the “light-use” of various plants. He took as his standard of unity for the higher plants the amount of light required to darken photographic paper in one second. When dealing with lichens he adopted a more arbitrary standard, calculating as the unit the average amount of light that lichens would receive in entirely unshaded positions. He does not take account of the strength or duration of the light, and the conclusions he draws, though interesting and instructive, are only comparative.