94 THE POPULAR SCIENCE MONTHLY 



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. Adventi- 

 tious 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 adventi- 

 tious roots in that case are not very numerous nor very long. Some- 

 times, 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. Per- 

 haps 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. 





