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W. A. CANNON 



tion. Such forms as Covillea and Franseria, on the other hand, 

 have root-systems which may either penetrate fairly deeply or 

 extend widely according as the soil is deep or shallow. Such 

 species, whose roots are thus capable of great variation, have 

 the widest possibilities, other conditions being equal, as to the 

 habitats they frequent. A third root type consists of a large 

 development of laterals, horizontally placed, and a poor devel- 

 opment of the tap root, or other roots capable of deep penetra- 

 tion. The cacti as a whole and plants of certain other families 

 with water storage organs have roots of this type. The roots 

 of the third type, whatever m&y be the depth of the soil, lie 

 relatively close to the surface, so that it is apparent that soil 

 depth is not of the first importance among those factors which 

 determine their position. It will be shown directly, however, 

 that root-systems of the third type are capable, in nature, of a 

 certain degree of variation. 



For the purpose of showing the extent of root variation to 

 be found in plants when growing under natural conditions it 

 will be instructive to note that observed in representatives of 

 the second and third types. The root-system of Franseria del- 

 toidea may be selected as representative of the generalized form 

 of root, the second type, and that of Echinocactus wislizeni as 

 typical of the third root type. 



Franseria occurs on the domain of the Desert Laboratory on 

 the slopes of Tumamoc Hill and on the subaerial fan which ex- 

 tends eastward from the Tucson mountains. Occasional speci- 

 mens are also to be found along the flood-plain of the wash which 

 lies west of Tumamoc. The soil conditions of the first two areas 

 are nearly alike, that is, the depth available for the roots of 

 plants is usually under 50 cm., while the soil by the wash is 2 

 m. or more in depth. In the first two areas an impervious 

 hardpan underlies the superficial soils. The root-system of Fran- 

 seria on the areas having shallow soil consists of a short tap root 

 and laterals which arise at a depth of about 12 cm., which may 

 reach outward 1.6 m., and which may attain a depth of 12 to 

 36 cm. Specimens of the same species, but from the flood-plain, 



