752 



SCIENCE 



[N. S. Vol. XL. No. 1038 



is, on the hamada or its equivalent, and not 

 the former, from which it follows that the 

 generalized type of root-system is really the 

 xerophytic type par excellence^ and not the 

 type with the most deeply penetrating tap-root, 

 as might be supposed." An interesting case 

 of accommodation of root habit to character 

 of the soil is mentioned : " The roots of 

 Haloxylon on the hamada at Ghardaia develop 

 both laterals and a main root, but in deeper 

 soil, as at Biskra and Ghardaia also, the 

 laterals are nearly suppressed and the tap-root 

 is the striking feature." 



The Algerian desert vegetation was found 

 to have been greatly modified by grazing. In 

 the vicinity of large towns, such as Ghardaia, 

 the cemeteries, which are surrounded by walls, 

 were practically the only places where the 

 native vegetation could be found in a rela- 

 tively undisturbed condition. The author 

 comments on the fact that certain species, 

 Haloxylon articvlatum, for example, which 

 are persistently grazed and of which the dis- 

 semination would appear to be very difficult, 

 nevertheless remain extremely abundant. It 

 is pointed out that this factor must have been 

 operative even before domesticated animals 

 were introduced into the region, since the na- 

 tive fauna includes several grazing animals. 

 A striking indication of the modifying influ- 

 ence which the persistent action of this factor 

 during many centuries must have had upon 

 the vegetation is afforded by the present dis- 

 tribution of the betoum (Pistacia atlantica) : 

 " The betoum, which is the largest arboreal 

 species in the Sahara, is confined to the region 

 of the Dayas; that is, to the country immedi- 

 ately south of Laghouat. The tree is un- 

 armed and is eagerly sought after by all her- 

 bivorous animals for its foliage and tender 

 twigs. Owing to the presence of such animals, 

 wild and domesticated, the young tree would 

 have no chance to survive were it not that, 

 growing in association with it, is the jujube 

 (Zizyphus lotus), which is armed and is not 

 eaten by any animals. The jujube affords safe 

 protection for the seedling betoum, and in its 

 capacity as nurse prevents predatory attacks 

 by animals during the critical period. The 



survival (and probably the distribution as 

 well) of the betoum is mainly conditioned on 

 the presence of its protector." 



At Ghardaia it was observed that many of 

 the perennial species were resuming growth 

 and beginning to flower in November, although 

 no rain had fallen for twelve months. The 

 following explanation is suggested : " Judging 

 from analogy, therefore, it would appear that 

 the stimulus to development on the part of the 

 M'Zabite plants may be from the relatively 

 better water relations made possible by a lower 

 temperature without rain. In November at 

 Ghardaia 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 sea- 

 son. 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 con- 

 ditions favorable for their appearance." 



Exposure appeared to be an important factor 

 in plant distribution only near the northern 

 edge of the desert. 



"In parts of the Sahara visited where the 

 most rain is reported, especially Laghouat and 

 Biskra, plants were observed to exhibit expo- 

 sure preference. Here the south or southerly 

 facing slopes may have a floral composition 

 different from the opposite exposure. In each 

 instance the soil conditions, and apparently 

 the moisture conditions also, were alike." 

 Farther south, at Ghardaia, " provided there 

 is sufficient depth of soil, apparently any spe- 

 cies may be found on any exposure." 



The numerous excellent illustrations show- 

 ing the general appearance of various types of 

 vegetation and the habit and root development 

 of characteristic species are an attractive fea- 

 ture of this publication. 



