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.