ROOT HABITS OF DKSERT PLANTS. 



39 



leading characteristics of its root-s}stem are the long tap root and the small 

 development of laterals. The tap root of the young plant may reach as 

 deep as 13 cm. before laterals appear to any extent (compare plate 7), and 

 even in the mature form there are few laterals, and these branch but little. 

 The extreme depth to which the tap root of the mature plant reached was 

 not learned, but it may safely be considered among the most deeply placed 

 of the roots of any annual. 



Rafinksouia Neo-mkxicana. 



Rafincsqnia occurs on the bajada at the northern base of Tumamoc Hill 

 and on the hill itself. Its root-system is characterized by a prominent tap 

 root which bears filamentous laterals. The tap root is 2.5 mm. in diameter 

 at the crown. The laterals of the first order are 5 cm. long, more or less, and 

 branch infrequently; laterals arise singl}^ and cultures (to be mentioned 

 below), in which attempt was made to induce the formation of laterals 

 secondarily and in groups, were not successful. In Rafinesqnia, therefore, 

 the appearance of laterals singly should be considered a fixed character. 



Streptanthus Arizonicus, Sisymbrium Canescens, and Sisymbrium 

 Reflexum. 



The three cruciferous annuals whose root-systems were studied occur 

 both on Tumamoc Hill and the upper reaches of the bajada at its northern 

 base. Streptanthus, however, appears to be limited to the Hill. Streptan- 

 thus and Sisymhrimn have similar root-systems. The tap root is a prom- 

 inent feature, and the laterals are fine and long, and branch but little. The 

 roots thus are extensive rather than intensive, enabling the plant to reach 

 out as well as to penetrate the ground a considerable distance. Plate S 

 shows well the general character of the young and of the mature plant of 

 Streptanthus and does not need further comment. 



A specimen of Sisymbrium with the shoot 15 cm. long and fruiting was 

 examined. The tap root was relatively heavy and was traced over 16 cm. 

 into the soil. The laterals of the first order were either fine or filamentous 

 and were 10 cm. or more in length, and were borne for the most part within 

 2 to 6 cm. of the surface of the ground. 



Both in Streptanthus and in Sisyinbriuni the laterals arise singly; there 

 are no groups of laterals and no rudiments. 



