74 



ROOT HABITS OF DESERT PLANTS. 



Specimen. A tap root was traced down 40 cm. and gave off, within 15 cm. 

 of the surface of the ground, 6 main laterals. These reached out less than i 

 meter from the main root axis, and certain of them were inclined down- 

 ward at a rather sharp angle. The laterals gave off frequent branches of 

 two sorts, filamentous as described for the other specimen, and relatively 

 large ones. In addition to the main laterals and their branches, there 

 were very numerous slender roots close to the surface and taking their 

 origin also from the tap root. The latter type of roots was entirely lacking 

 in the first plant examined. The contrast between the two plants in this 

 regard is shown, but not adequately, in plate 19. 



Fig. 13. — Root-systems of Riddellia cooperi. 

 ij. Horizontal extension of root-system which was growing iu the sandy loam cast of West Wash., 

 showing the most prominent laterals but none of the large number of slender ones which were 

 at base of shoot. 



b. Vertical extension of plant shown in a. Tap root penetrated 1.44 m. beneath surface. 



c. Horizontal extension of root-system of a specimen which was growing west of the wash in clay 



soil. Dotted line shows position of an intruding root of a neighboring Acacia consiricia. 



In considering the form of the root-systems of the two plants, both hav- 

 ing a relatively short tap root, though the systems were such as to lead one 

 to expect a deeply penetrating root, it was decided to excavate the roots of 

 another plant with the view of observing especially the behavior of its tap 

 root. Accordingly, a third plant was selected, growing east of the wash 

 and close by the first specimen studied. The soil conditions where the sec- 

 ond and third specimens examined were growing were apparently the same. 

 The shoot of the last plant studied was 38 cm. high. The root-system of 

 this specimen Avas dominated by a tap root, which was traced to its end. 



