72 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



arbitrary, since the climate is a complex of various factors and its effects 

 on vegetation are also complex. Among the climatic factors whose effects 

 are most striking are the air temperature and the rainfall, and only certain 

 effects resulting from a variation of these will be commented on here. 



In parts of the Sahara visited where the most rain is reported, especially 

 Laghouat and Biskra, plants were observed to exhibit exposure preference. 

 Here the south or southerly facing slopes may have a floral composition 

 different from the opposite exposure. In each instance the soil condi- 

 tions, and apparently the moisture conditions also, were alike. Exposure 

 preference was not noticed in the southern portions of the colony. Another 

 temperature relation was observed, namely, the renewal of growth in the 

 autumn. This is probably direct effect, although the point is not certain. 

 In Ghardaia it was seen that many of the perennials were taking on new 

 growth and coming into flower, although no rain had fallen for 12 months. 

 Analogous conditions, with a significant difference, occur each year in the 

 Tucson region. Here with the change from a cooler to a warmer tempera- 

 ture, as from winter to spring, or from spring to summer, fairly independent 

 of the rainfall, many perennials organize flowers or shoots. But, so far as 

 is known, no species renews its vegetative activities with the coming of 

 winter, or with a decreased temperature and also independent of the rains, 

 although there are characteristic winter and summer plants. Judging from 

 analogy, therefore, it would appear that the stimulus to development on the 

 part of the M'Zabite plants maybe from the relatively better water relations 

 made possible by a lower temperature without rain. In November at Ghar- 

 daia the evaporation rate is much below that of summer, that during the 

 night being very small. Further, it was told me by good authority that the 

 same species seen growing in autumn renew growth whenever rain chances to 

 come, whatever might be the season. But it should be remembered that 

 rain most commonly occurs in this region in winter, so that the plants may 

 have a rhythm to which they usually conform, but from which they may 

 depart, and that both stimuli (better water relations and lower temperature) 

 are the annually recurring factors by which it may have been induced. 

 Reference, of course, is made to perennials only, as no annuals were seen 

 until the rains of spring made conditions favorable for their appearance. 



The effects of a varying amount of precipitation are naturally the most 

 marked of any climatic factor. It is especially striking as one goes south 

 from the Mediterranean, crossing the Tell and the High Plateau and enter- 

 ing the desert proper. Whether the effects would be increasingly striking 

 with deeper penetration of the desert is doubtful. As is well known, a lead- 

 ing characteristic of the vegetation of the littoral and of the Atlas Moun- 

 tains is the presence of forests of whatever species. As the littoral is left 

 behind the forests disappear until on the High Plateau there are only strag- 

 gling trees along the dried water-courses. This steppe bears mainly shrubs, 



