52 ROOT HABITS OF DESERT PLANTS. 



paragraph. The entire habit of the plant was symmetrical and loose. 

 Figure 5 and plate 10 show the extension of the roots in the ground and their 

 appearance when removed. There was a tap root, which penetrated 15 cm. 

 and which bore a few laterals not over 10 cm. long, and about 4 main laterals 

 3 of which extended from the main axis as far as 30 cm. and varied in depth 

 from 5 mm. to 5 cm. All of the laterals were slender and bore almost no 

 branches. Thus the root-system was very meagerly developed. 



The specimen which was growing apart from other plants was 50 cm. 

 high, of a compact growth-habit, and was much branched. The root- 

 system of this plant may be briefly described as follows. There was no well- 

 defined tap root, but there was a brush of slender roots varying much in 

 length. The roots radiated out from 60 cm. to 2.8 meters and the longest 

 was less than i cm. in diameter at its base. The depth attained by the 

 longest roots ranged from 4 cm. to 14 cm., with the usual depth between 

 5 and 8 cm. The roots branched hardly at all. 



In contrasting the root-systems of the two specimens it will be seen that 

 that of the protected plant is composed of fewer and shorter roots than that 

 of the unprotected one, and that the roots of the former are also nearer the 

 surface of the soil. In either instance it is probable that the character of 

 the root-system, at least as far as regards the number and the length of the 

 roots, is immediately connected with the development of the shoot, and 

 that the character of shoot development and the vertical position of the 

 roots are intimately associated with the protected or unprotected condi- 

 tion of the plant. 



Opuntia Versicolor. 



Opuntia versicolor, one of the cylindro-arborescent forms, is one of the 

 best represented species on Tumamoc Hill, although in no place does it 

 occur in large enough numbers to constitute the dominant plant type. The 

 habitat where the plants studied were growing was in the main similar to 

 that described for Encelia farinosa, although it differs in certain details. 

 The upper malpais soil is relatively deep and carries, on its surface and 

 embedded in it, stones and boulders in some abundance. The water con- 

 ditions are better than where Encelia grew, owing to the nearly level char- 

 acter of the ground, the rather deep soil, and also to the fact that the place 

 receives considerable water as superficial run-off and as seepage from higher 

 portions of the hill. 



Two specimens of versicolor were studied, of which one was young and the 

 other mature. The younger plant was less than 50 cm. high and had few 

 branches. There were two well-defined root types in the plant, namely, 

 those shallowly placed and extending horizontally, and those extending 

 vertically and reaching rather deep. The latter, the anchoring or sup- 

 porting system primarily, consisted of a tap root, with short and long lat- 

 erals, M^hich penetrated to a depth of 20 cm. One branch of the tap root 

 attained a depth of 30 cm. The superficial portion of the root-system was 



