The general principles bearing upon the relation between the type of root-system and the distribution of the species, as observed in southern Algeria, can be briefly stated. Often the relation is close and apparent, but not always. For example, large perennials, such as _Tamarix_ and _Zizyphus_, have an obligate specialized root-system, with a long tap-root. These plants naturally occur where there is considerable depth of soil, and hence are not to be found on the hamada, for instance, where it is shallow. On the other hand, such species as have a generalized root-system, like _Acanthyllis_ and _Haloxylon_, are to be found on the hamada, but they occur also in other habitats where the soil is deep. The last type of root-system is flexible, accommodating the species to a wide range of soil conditions. In doing this the change in form is almost a change in type; for example, the roots of _Haloxylon_ on the hamada at Ghardaia develop both laterals and a main root, but in deeper soil, as at Biskra and Ghardaia also, the laterals are nearly suppressed and the tap-root is the striking feature. A marked exception to the rule that plants with a generalized type of root-system have also the widest local distribution lies in _Peganum harmala_, which, having roots of this character, is nevertheless restricted to habitats where the soil is deep. The species is a half-shrub, having a perennial subterranean portion and a short-lived subaerial portion, the life of which appears to depend on the character of the water-supply. As learned by Fitting, the species can develop in its leaves a very dense sap, enabling it to extract water from a very dry soil.[37] In spite of this fact it appears to act like an annual in certain regards, requiring at all times, particularly during the most arid season, a relatively good water-supply. Thus, in brief, a study of the relation of the root-type of the Algerian plants to the plant’s distribution leads to the same general conclusion already obtained by similar but more extended study in the Arizona desert, namely, that the connection is often a very close one and often of definitive importance. Where the root-type is an obligate type the distribution of the species is much restricted, but where it undergoes modification with changed environment the distribution of the species is much less confined. It is of interest to note especially that as a rule it is the latter kind of root-system that is developed by such plants as occur where the soil conditions are most arid, that is, on the hamada or its equivalent, and not the former, from which it follows that the generalized type of root-system is really the xerophytic type _par excellence_, and not the type with the most deeply penetrating tap-root, as might be supposed. THE BIOTIC FACTOR.