76 ROOT HABITS OF DKSIvRT PLANTS. 



coarse. The older ones wcR'shrivckd, and of a dark red color, but the voung 

 roots, which were put out from the base of the plant and not as branches of 

 already existing roots, were stout and pinkish. The young roots ended 

 abruptly and very evidently functioned more as storage than as absorbing 

 organs. No tap root was seen in any specimen of Agave examined. 



The bases of the leaves and the axis of the stem of Agave are fleshy and 

 ser\-e the purpose of storing both water and food-stnfTs against the flower- 

 ing season or other periods of need. The other plant studied at this time, 

 which has storage facilities above the ground, Dasylirion iexanum, has, as 

 will appear at once, a similar root-system. 



Turning now to consider Dasylirion and Yucca and the habitats in which 

 they were growing, we shall find, as revealed by the character of the vege- 

 tation as well as shown by soil differences, that the bajada farther from the 

 mountain base does not have uniform conditions. For example, in the 

 vicinity of Dasylirion there are Acacia constricta, Bigelowia hartwegii, Opun- 

 tia blakcana, Ridddlia coopcri, Yucca sp. Besides these species, in the 

 vicinity of the Yucca sp. studied, the following occurred: Parkinsonia 

 microphylla, Parkinsonia torreyana, Prosopis velutina, and Zizyphiis parryi. 

 At lower altitudes, as at Tucson, these species do not occur together, but 

 are characteristic of the dilTerent well-defined physiographic areas; for 

 example, Parkinsonia torreyana grows only in the vicinity of washes, or 

 where the water conditions are relatively favorable, while P. microphylla 

 occurs on the bajada above the wash. Both Zizyphus and Prosopis are 

 chiefly found on the flood-plain, occurring not at all, or as dwarfed speci- 

 mens onlv. in the other and more arid areas. 



In addition to the differences in the two habitats last considered, as 

 revealed bv the difference in the kinds of woody plants growing in them, the 

 Yucca sp. habitat has far the larger population. Without further investi- 

 gation it is assumed that this difference is wholly due to the fact that the 

 latter habitat has deeper soil, and hence more water and for a longer time, 

 than the alternate one. The root-systems of the plants growing in the first 

 habitat, Dasylirion and Yucca radiosa, will be first described. 



The specimen of Dasylirion examined was a meter high. The roots 

 W'ere exposed on the southern side only and had the following character- 

 istics: A large number of roots, each about 5 mm. in diameter, formed a 

 cluster at the base of the stem. The roots ran downward at an acute angle 

 and also extended out into the soil in a more or less horizontal direction. 

 The roots were all coarse ; the most shallow were 1 5 cm. beneath the surface 

 and the deepest about 36 cm. deep. The longest roots were the most super- 

 ficial ones, of which some measured 2.25 meters, though they were mostly 

 shorter than this (plate 19c). 



.Another specimen of Dasylirion, with the shoot about 2 meters high, 

 possessed a root-system essentially like the smaller plant just described, 

 although the soil at the place was somewhat deeper than that in which the 



