50 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



unexpected in view of the fact that no rain had fallen in the region for 

 twelve months. A study of the roots in relation to the depth to water 

 showed also that most of the native plants, during the dry season, could 

 not have penetrated to a depth anywhere near that of the water-table of the 

 valley, and the water relations of the plants growing on the plain above the 

 M'Zab Valley were even more severe. Further, most of the species do not 

 possess water-storage faculty. It should be noted that the only apparent 

 difference in the water relation between November and in early autumn, 

 or summer, was that of raising the relative humidity of the air through the 

 lower temperature. In a preceding section it has been stated that little 

 or no evaporation took place at night in November at Ghardaia, and pos- 

 sibly less in the daytime than would have been supposed. The leading 

 environmental changes were, of course, the lower daily temperature and 

 the really cool nights. 



It is not uncommon in the Arizona desert for a species to form flowers 

 or leaves, following a change in temperature, the moisture conditions being 

 not otherwise changed, but, so far as I know, the temperature changes 

 bringing about this result are always from a cooler to a warmer condition 

 and not the reverse. It would not be expected, consequently, that in the 

 present instance a renewal of vegetative activities would follow as a direct 

 result of a lower temperature, although analogous changes are necessary 

 before certain species, after rest, will start development. Whatever may 

 be the immediate cause of the renewal of activity on the part of the plants 

 at Ghardaia as noted, the following species were seen to have formed new 

 leaves: Henophyton deserti and Zilla macroptera; also the following fresh 

 flowers with or without shoot growth : Fagonia bruguieri, Haloxylon articu- 

 latum, Henophyton deserti, Ononis polydada, and Zillikoferia resedifolia, and 

 three other species not determined. 



GHARDAIA TO TOUGGOURT. 



From Ghardaia the route, consisting of camel trails only, pursued a 

 course south of east to Ouargla, and from Ouargla a direction east of north 

 to Touggourt, over 400 kilometers, Ouargla being about half-way. There 

 are no villages between Ghardaia and Ouargla and none between this place 

 and a point 20 kilometers south of Touggourt, so that in accounts of the 

 vegetation or the topography, lacking convenient points around which to 

 center descriptions, the device will be adopted of using distance estimations 

 to or from the three chief towns. 



The trail followed the valley of the Oued M'Zab, or kept close to it, for 

 63 to 73 kilometers before finally leaving it. It passed the sister towns of 

 Ghardaia and crossed short intervals of plain, descending occasionally to 

 the oued. The walls of the M'Zab Valley, 60 meters more or less at 

 Ghardaia, become lower and less precipitous as one goes down the drain- 

 age, until at length they become little more than rounded banks. The 



