which grows in the bottoms, or along the sides of the bottoms, in the vicinity of Tucson. The young plants have roots which strike directly downward, giving off almost no laterals within one meter of the surface of the ground. The depth to which the tap root attains has not been determined. As the plant becomes older a sucker is sent out close to the surface, from which there springs up a daughter plant. Adventitious roots occur along the course of this sucker, particularly where the daughter shoot arises. Occasionally the connection between the daughter shoot and the mother plant is not destroyed, and the adventitious roots in that case are not very numerous nor very long. Sometimes, however, the connection between the plant and offspring is broken and the adventitious roots, or one of the adventitious roots, strike straight down and behave precisely as the main root of the parent plant. That is, in this case, as in the Zizyphus and Tamarix, the root-system is an obligate deeply penetrating one, for which reason the species is confined to such localities as provide sufficient depth of earth.
The third type of root-system, which may be called a generalized type, is such as is possessed by most of the plants growing in the vicinity of the Desert Laboratory, and in fact by most of the desert plants. Perhaps it would be clearer to state this in another way, namely, that the plants which cover the greatest area in the arid region are such as have the generalized type of root-system. It will only be necessary to refer to the root-system of the creosote bush of the southwest for an example of this type. The roots of the creosote bush extend outward from the main stem for a distance of about three meters, less in small plants, and reach downward, either directly or at an angle, to a depth which is usually determined by the character of the soil. On the mesa, where the soil is usually less than one half meter in depth, the roots of the creosote bush do not exceed that depth, but in the beds of the washes, or rather on the flood-plains of the washes, where the soil is deeper, they have been known to attain a depth of over two meters. From this it is seen that the generalized type of root-system is more flexible than either of the other two types given, and it follows, other conditions being equal, that species with the generalized type of roots may also have a wider local distribution.
It is interesting to note that the most arid portions of an arid country are the areas which are above the flood places of the washes. In the southwest these are usually the mesas. In southern Algeria, for instance, these excessively dry areas are the regs, or the hamadas. It is to lower-lying areas, washes and the flood-plains of the washes that drainage from the higher ground flows, and also where particles of soil from the higher ground are deposited through water or through wind action. And the result is that the low-lying areas have deeper soils and more water than the upland.