GHARDAIA. 45 



From these observations it would appear that the roots of most of the 

 plants growing on the plain have a well-developed main root, and that few, 

 if any, perennial roots lie near the surface of the ground. This condition 

 is rather different from that seen at Laghouat, where the root-system of 

 typical inhabitants of a similar habitat is of the strictly generalized type 

 and may point to a difference in some character of the habitat — for instance, 

 the precipitation at Ghardaia.* 



In certain portions of the floor of the M'Zab Valley the soil conditions 

 favor full and normal development of all forms of root-systems, but in 

 other portions, because of the presence of an impervious hardpan, such 

 development is not possible. Where there is hardpan the presence of 

 species with obligate tap-roots is precluded, while such as have a more 

 flexible root-system (e.g., Peganum harmala) can to a degree accommodate 

 themselves to the unfavorable soil conditions. However, one instance was 

 seen, which will be reported below, where a plant with a tap-root was found 

 growing on a hardpan stratum so hard as to be excavated with the greatest 

 difficulty. The character of the root is very greatly modified by the pres- 

 ence of the hardpan. Following is a sketch of the root-systems of a few 

 plants growing naturally in the valley. 



Peganum harmala is one of the most characteristic plants of the floor of 

 the M'Zab Valley; so far as my observations go, it usually occurs where 

 there is much soil and where the water relations are the most favorable. 

 Several studies of it were carried out on plants growing in different parts 

 of the valley. In order to make the leading study of its roots, a typical 

 habitat was selected east of Ghardaia and nearly in the midst of the 

 valley. Here the upper soil, to a depth of about 20 cm., is a fine sand 

 carrying water-worn pebbles and coarse stones. Below the sand is a denser 

 stratum a few centimeters in thickness, but not the hardpan described in 

 another place. Below the harder stratum, to an undetermined depth, are 

 mingled sand and gravel coarser in texture than the superficial sand. 



The first specimen of Peganum studied in this habitat was about 30 cm. 

 high and in full leaf. Its roots were fibrous, that is, they were not fleshy. 

 The root-system consisted of a main root, which ran directly downward 

 17 cm., and several laterals. Probably owing to the influence of the hard 

 stratum, the main root at length turned abruptly and maintained a hori- 



* It has already been shown that the number of days on which rain may be expected 

 to fall each year is greater at Laghouat than at Ouargla, and probably at Ghardaia also. 

 According to reports, the rainy days at Laghouat vary from 20 to 84 (seven years' 

 observation), with an average of 49 each year. The average number of days on which 

 rain falls at Ouargla is 14.2. The amount of rain at Laghouat is 200 mm., at Ouargla 

 90.2 mm. It would appear, therefore, that the average rain at Laghouat is less in 

 amount than the average rain at Ouargla; or, in other words, it points to the torrential 

 as being the type of the desert storm. Since, other things being equal, the greater storms 

 would penetrate the ground the most deeply, we may here have an explanation of the 

 emphasis at Ghardaia on the tap-root as against the generalized root as the type of 

 the root-system. 



