46 ROOT HABITS OF DESERT PLANTS. 



The relation of the specimen of Echinocactus from Tumamoc Hill to other 

 perennials included within the radius of its horizontal roots was as follows : 

 One specimen of Acacia const ricta was placed i meter from the main axis of 

 the plant ; one specimen of Op^mtia discaia was slightly farther than Acacia ; 

 and ID specimens of Mcnodora scabra, a shrubby form, occupied various 

 positions nearer than the other species named. The neighboring forms, 

 however, held very unlike relations to the Echinocactus, a fact, in this case, 

 dependent on the character of the root-systems. The roots of Menodora 

 penetrated deeply and there was little development of the laterals corre- 

 sponding to the horizontal or absorbing system of Echinocactus. Only a few 

 of the roots oi Acacia were encountered while excavating for the root-system 

 of Echinocactus, but two long and a few short roots of Opuntia were seen 

 to lie in close proximity to the downhill roots of Echinocactus. From the 

 positions occupied by the roots of these plants it may be concluded that 

 only the species of cactus enter into active competition with one another 

 for water. Competition between plants whose roots occupy horizons 20 to 

 30 cm. apart, as between the cacti and other plants, is not to be con- 

 sidered as direct. 



The story of the developmental changes which take place in the root- 

 system of Echinocactus was not learned, although it is undoubtedly different 

 from that of such a fleshy form as the sahuaro (Carncgiea gigantea) in which 

 the horizontal portion of the root-system becomes modified secondarily to 

 meet the demands of the plant for mechanical support, as will be described 

 below\ The root-system of Echinocactus is not suited to enable the epigeal 

 portion to withstand severe lateral strains, as is the case with the sahuaro, 

 for which reason the plants are often found uprooted, and are usually 

 easily overthrown by a well-directed push. It is a matter of common 

 observation that in falling, or in leaning, the plant generally leans or falls 

 in a southerly direction. It thus may be called the compass-plant of the 

 desert. The consistence of this position is without doubt founded on 

 normal developmental changes to which the plant is subjected, such as the 

 greater growth of tissues on either the north or the south side, as in the 

 sahuaro (E. S. Spalding: Mechanical adjustment of the sahuaro, Cereiis 

 giganteus,, to varying quantities of stored water. Bull. Torr. Bot. Club, 

 vol. 32, page 64, 1905), by which an asymmetrical distribution of weight 

 takes place. 



Encelia Farinosa. 



Encelia has a very decided habitat preference. Not only is the species 

 limited to Tumamoc Hill, but it is most abundant on the southern face of 

 the hill, though occurring also on the west and east, and to a limited extent 

 on the north side. As the most t5^pical habitat was not favorable for exam- 

 ination of the roots of the plant, on account of the rock exposures and the 

 steep gradient, it was studied on the northern slope where the soil is 

 deeper and the roots more easily removed. 



