GENERAL SUMMARY AND CONCLUSIONS. 75 



a drought for over twelve months, but on the return of the cool season, 

 with a lower evaporation rate, growth was resumed and several plants 

 came into flower. This appeared to be not wholly the stimulus of lower 

 temperature, since I was informed by good authority that the plants 

 renewed their various activities whenever rains chanced to come, what- 

 ever might be the season. 



From the observations last given it appears that sufficient moisture per- 

 sists in the soil to tide perennials over the long periods of drought, although 

 not in sufficient amount to permit active growth during the dry seasons. 

 This is not an uncommon occurrence with desert plants. For example, 

 whenever in the Tucson region the arid seasons are uncommonly long, or 

 there has been a relatively small rain, as 25 per cent less than usual, much 

 of the vegetation may remain dormant. Under such conditions an ever- 

 green like Covillea tridentata drops all save the youngest and smallest leaves 

 and maintains this nearly defoliate condition for long periods, with little 

 other change. The plant behavior noted is always connected with insuffi- 

 cient moisture. 



The soil in the oueds and probably also in the reg, as well as that of the 

 dayas, is of considerable depth. The soil depth, or rather the depth before 

 solid rock is encountered in the valley of the Oued M'Zab at Ghardaia, 

 is from 20 to 30 meters, or even more; at the Daya Tilrempt the depth to 

 water in one of the wells is about 95 meters. Although there is much gravel 

 and sand in the fill of these depressions, it is likely that they afford the 

 most favorable soil conditions in the desert for the development of a large 

 root-system. Be that as it may, it is certain that large plants, such as 

 Tamarix and Pistacia, occur only in such places. 



Where there is most soil on the hamada, as in certain pockets near the 

 old hill town of Ghardaia, abandoned several centuries since, one finds 

 also the most and largest plants of this, the hamada, formation. Moreover, 

 at the time in November when growth was noticed in the oued plants, it 

 was also taking place in these favoring situations, about 50 meters above the 

 level of the valley of the M'Zab and much above any possible permanent 

 water-supply such as is afforded by a water-table. 



The various habitats, therefore, not including the oases, are naturally 

 closely associated with soil differences. These are few and, in nearly each 

 case, to name is sufficient definition. There are dunes (areg), hamada or 

 stony desert, reg or alluvial desert, the daya, and the flood-plain of the 

 oueds (reg?). Modification of the hamada, reg, and oued flood-plain occurs 

 whenever white hard-pan (caliche) is present. The soil of the reg, often 

 that of the oued flood-plain and that of the daya, is fine alluvial and is rela- 

 tively or actually deep. The hamada has the poorest soil condition, being 

 underlaid by rock, and often or always by hard-pan as well. Large stones 

 and boulders are embedded in the soil or lie on the surface. A modifica- 

 tion of the hamada occurs whenever sand is strewn over its surface, even 

 6 



