If we ask _why_ the plants group themselves into the associations which we may study any day in the country, in many cases the answer is not obvious. It is clear that while many species accommodate themselves easily to different soils or different degrees of light or of moisture, others have small powers of accommodation, and are in consequence restricted in their range. By long usage many plants have acquired special characters enabling them to live under special conditions--some examples will be discussed a little later--and in some such cases it is easy to correlate the peculiar characters of the plant with those of the habitat. But in many other cases the relation is not obvious. For instance, we cannot tell, by examining a plant, whether it is partial to a limy or to a non-limy soil; yet many plants are poisoned by lime, while others, though generally capable of growing in a soil devoid of lime (if planted in a garden), are nevertheless absent from the non-calcareous areas adjoining their limestone habitat; in other words, they can hold their own on limestone, but are unable to do so elsewhere. The two ferns already mentioned (_Polypodium Robertianum_ and _Lastrea rigida_) are cases of the latter kind; while some of the most familiar of our hillside plants, such as Foxglove (_Digitalis purpurea_) and Broom (_Sarothamnus scoparius_), are instances of the former. If, however, we consider some of the formations or associations which are the result of extreme conditions of environment, we get more light on the relations between the plants and the factors which control the vegetation. Take the case of the plants inhabiting desert regions such as were discussed in Chapter I. Here the outstanding feature is scarcity of water, and the plants display various remarkable adaptations which fit them for a thirsty life. There are three ways to meet scarcity of water--facilities for gathering it, arrangements for storing it, and economy in using it; and arrangements for all three are familiar features of desert plants. To effect the first, the root-system is extended, and is often enormously developed in proportion to the aerial parts. This adaptation may be studied in the flora of dry places in our own country, such as shingle beaches and sand dunes, which are characteristic semi-deserts. Take such plants as the Sea Holly (_Eryngium maritimum_), the Sea Convolvulus (_C. Soldanella_), or the Sea Sedge (_Carex arenaria_), and compare the extent of the root-system or underground stems with that of the aboveground portions. Fig. 4 represents the Wild Carrot (_Daucus Carota_) as found growing under extreme exposure on the west coast of Ireland. To meet the conditions the tall branched stem has been entirely dispensed with, and the terminal umbel is seated on the ground in the middle of a ring of leaves. In this way the plant prepares to resist both drought and wind. Water storage is often developed in different parts of xerophytes