26 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



The census of Acanthyllis was taken on the upper portion of the arid 

 plain at a place where the plant seemed to be most abundant. On an area 

 16 by 16 meters, 92 specimens were found living. This was the dominant 

 species. Other species, present in less number, were so badly eaten by 

 animals as to be quite unrecognizable. 



The root-system of Acanthyllis offered some points of interest and a short 

 study of it was made in the field. All of the specimens whose roots were 

 examined were growing in the habitat above referred to and within a meter 

 of one another. The leading results of the observation are as follows : The 

 largest specimen studied possessed a tap-root 3 cm. in diameter at the 

 crown. Growing rapidly smaller as it ran downward, the root gave off 

 four laterals, of which a portion dipped at an acute angle to a depth of 

 20 to 30 cm., sending off branches by the way. One of the largest of the 

 laterals was traced 75 cm. and where left was 2 cm. beneath the surface. 

 The branches, at least of the main laterals, that is, the roots of the tertiary 

 order, for the most of their course ran thus near the surface of the ground. 

 One of the leading laterals was followed to the base of a neighboring speci- 

 men of quedad, where it lay close to the crown of the main root. The 

 depth of the penetration of the tap-root of this specimen was not learned. 

 The tap-root of a neighboring specimen ran directly downward 20 cm.; 

 then, turning sharply, it extended in a horizontal direction for a distance 

 of 70 cm. As the large laterals were wholly lacking on this plant, the tap- 

 root was the entire system — surely an anomalous condition. On a third 

 plant the tap-root penetrated the ground about 4 cm. only, after which it 

 turned and ran the rest of its course within 4 cm. of the surface. The more 

 superficial of the roots of a fourth specimen were found to extend to the 

 base of the last plant mentioned. Thus it was found that the root-system 

 of Acanthyllis, as growing naturally, extends both widely and deeply for 

 a considerable distance, and that it is flexible to a degree; in short, is gen- 

 eralized* and closely resembles that of certain species of the Tucson region, 

 particularly Covillea tridentata, which grows under similar conditions. 



The root-systems of three or four other species were also examined. Of 

 these Zollikoferia spinosa and Artemisia campestris were growing in a little 

 hollow in the plain close by the habitat of Acanthyllis. Zollikoferia has a 

 very close habit of growth with dichotomous branching (compare figs. 5 

 and 7) . When dry the branches are slender and of a woody hardness. The 

 root-system of this species is characterized by a pronounced tap-root and 



* The Root Habits of Desert Plants. W. A. Cannon. Carnegie Institution of Wash- 

 ington Publication No. 131. 1911. This paper gives a descriptive classification of the 

 main root-types in the deserts of the southwest, in which such a root as found in Acan- 

 thyllis is called generalized, in distinction to roots like those of most of the cacti, or 

 Zizyphus, the former having a system wholly superficiaband the latter a system wholly 

 deeply placed, as specialized. The specialized root-systems appear to be so fixed in 

 character as to be not easily changed, while the generalized type is flexible. It will be self- 

 evident that the type of root-systems may be of great importance in determining the 

 local distribution of a species. 



