64 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



75, and 77). There were also a few specimens of Acanthyllis numidica, and 

 many annuals, which, unlike those on the south facing, were not in flower. 



On the lower slopes, in the small and open gulches and the lower por- 

 tions of the washes and their flood-plains, the plants were most numerous 

 and include a variety of forms, with Peganum harmala the most conspicu- 

 ous and most numerous. 



The roots of several species growing on the hills were studied with the 

 following as perhaps the leading results: The root-system of Haloxylon 

 scoparium, as has already been seen to be the case with an allied species 

 in the country of the M'Zab, may be said to be a modification of the gen- 

 eralized type. One specimen, partly exposed by erosion in a wash, had a 

 tap-root over 1 13 cm. in length (fig. 76) . As this root was 4 mm. in diameter 

 where left, and was 8 mm. in diameter at the crown, it may have penetrated 

 much beyond the point where it was left, provided the soil conditions con- 

 tinued favorable. The main root gave off two large laterals, of which one 

 left the parent root 8 cm. and the other 18 cm. beneath the surface of the 

 ground, but there were also numerous filamentous roots about 2 cm. in 

 length, which were borne in tufts. (See fig. 79-) These resembled the 

 deciduous rootlets found on many perennials in the Tucson region and 

 doubtless function quite as they, namely, they are organized at the begin- 

 ning of the rainy season, operate to increase the water-absorbing area of 

 the plant quicldy and greatly, and die as soon as unbearably arid condi- 

 tions set in. 



In the same wash, where the soil was deep with an admixture of small 

 stones and pebbles, the roots of other species were also examined. One of 

 these was Peganum harmala, whose roots were examined at Ghardaia. The 

 root-systems of the plant in the two regions were similar in being gener- 

 alized. A main root was found extending downward over 61 cm. and 

 it gave off three good-sized laterals, arising from 15 to 27 cm. beneath 

 the surface. The uppermost lateral took a horizontal course. Figure 82 

 gives a fairly good idea of the general character of the root-system of the 

 species. 



There were many specimens of Plantago albicans growing in the same 

 habitat as Haloxylon and Peganum. Its root-system was also studied with 

 the following results : The tap-root is strongly developed. In one instance 

 the slender main root was found to go straight down over 71 cm. Numer- 

 ous laterals were borne between 8 and 20 cm. beneath the surface of the 

 ground. The species has an interesting habit of propagating vegetatively 

 by means of fleshy stolons. An examination of the stolons showed them 

 to be of very unequal age, some having been lately formed, while others 

 had been organized in previous years and were no longer living. Many 

 specimens were examined to learn the probable service of the fleshiness of 

 its stolons, and the conclusion was that this factor probably enables the 

 species to pass over periods of excessive drought. 



