GENERAL SUMMARY AND CONCLUSIONS. 79 



But finally there were found two classes of plant formations in which 

 animals either had not intruded at all, or not to a harmful degree. One of 

 these is the wide flood-plain of the Oued M'Zab, or one of its tributaries, 

 lying about 50 kilometers east of Ghardaia. Despite the fact that the flocks 

 are very numerous in the vicinity, and that the flood-plain is on the regular 

 caravan route between Ghardaia and Ouargla, there are few signs of grazing. 

 The entire plain is so well covered by shrubs that the vegetation gives the 

 tone to the landscapes — a rare thing in the desert. Here one finds Retama 

 retam, Genista saharce, and Ephedra sp., as well as other species in fair 

 abundance. Thus the plants are not only numerous, but are of a good 

 size. It should be observed that on either side of the flood-plain, on the 

 hamada, scarcely any vegetation may be found. 



The second formation referred to is that of protected areas, especially 

 at Ghardaia, which have been little disturbed at any time, and portions 

 of them not at all disturbed for centuries. These are the cemeteries. Such 

 areas are situated not only in the floor of the M'Zab Valley, where the 

 plant conditions are relatively favorable, but also on the hamada, where 

 they are relatively very unfavorable. Both in the valley and on the hamada, 

 as well as on the valley wall between the two wherever there chances to 

 be soil, the plants are relatively numerous and of fair size. This fact has 

 been detailed under the section on Ghardaia and need not be more than 

 mentioned here. 



From these two general observations (exceptions to the usual conditions) , 

 that on the flood-plain of the Oued M'Zab and the cemeteries at Ghardaia, 

 it is concluded that the grazing of animals has had a very marked influ- 

 ence in modifying the flora of southern Algeria. So far as could be told 

 from the limited opportunity to observe, the modification has gone along 

 on at least two lines, which are, of course, closely related. The size of the 

 plants eaten is much under normal for the particular locality, and at the 

 same time the capacity of the plant for reproduction has been greatly 

 lessened. That such species as are not touched by animals have not spread 

 more rapidly, or even have not become the dominant forms, is an interest- 

 ing problem, and one that would have to be worked out for each species. 

 It can be suggested, however, that the restricted distribution probably lies 

 in the fact that the struggle of desert plants is mainly with an adverse 

 physical environment rather than with one another, and that such condi- 

 tions would not be affected by grazing animals. For example, large shrubs 

 do not occur on the hamada, hence Tamarix, Zizyphus, and Rhus, as well 

 as Peganum harmala, are limited to situations where the soil is fairly deep 

 and the water relatively favorable. Euphorbia guyoniana also occurs only 

 in sandy soil, and the number of plants limited in their distribution to soil 

 characters is necessarily a large one. 



