It is not everywhere even on our hospitable, habitable globe that conditions are suitable for plant growth. The reader will remember that the flat summit of Farleton Fell, where in fancy we still stand, was devoid to a great extent of vegetation; and that the sea-sands and mud-flats out to the westward presented a surface from which plants appeared to be absent. This question of _deserts_--that is, of areas of the Earth’s surface where the prevailing mantle of vegetation is wanting--is an interesting one, and may fittingly detain us for a few minutes. Deserts are produced by the failure of one or more of the conditions which are necessary for plant life. The factors in question may be briefly defined as _temperature_, _light_, _water_, _atmosphere_, and _mineral salts_. The majority of the higher plants have developed a complicated root-system for the purpose of collecting water (containing salts) from the soil, and of anchoring the organism firmly in its chosen abode, so a _soil_ is also usually essential. Here on Farleton Fell soil is missing over much of the surface, which is occupied by naked limestone rock. The absence of soil is due to the fact that the material--carbonate of lime--of which the rock is composed is soluble in water, unlike, for instance, the materials of which slate or sandstone rocks are composed; the rains slowly dissolve it, and it passes in solution down through crevices in the strata, leaving behind only a small insoluble residue. This residue, where not also washed away, collects in every little hollow, and lowly plants such as Algæ and Mosses soon discover it and colonize it. Their decayed remains add nutritive material to the little pocket, and help to retain water, and thus prepare the way by degrees for higher forms of life; till at length the crevices become filled with a luxuriant vegetation which, as we shall see later, is of a rather peculiar type. It should be noted that even the bare rock is not so inhospitable as completely to exclude plant life. If we examine it with a lens we shall see that it is colonized by minute Lichens, many of which have the power of dissolving the limestone, producing tiny burrows in which they live securely. [Illustration: FIG. 1.--A BURROWING LICHEN (VERRUCARIA CALCISEDA), LIVING IN LIMESTONE. _a_, Natural size; _b_, greatly enlarged; _c_, section, greatly enlarged.]