36 ROOT HABITS OF DESERT PLANTS. 



characterized by few roots which are rather coarse and which branch 

 infrequently. In the young plant the tap root predominates, but as it 

 matures the laterals become important ; the behavior of the root-system in 

 its development recalls that of Malva. Tubercles were not present on 

 the plants examined. Plate 6, sHghtly less than life-size, shows the main 

 features of the root-system so that further description in this place is not 

 necessary. 



Mentzelia Albicaulis. 

 Mentzelia albicaulis occurs on Tumamoc Hill and on the upper bajada- 

 slopes. The leading character of its root-system is the prominent tap root. 

 The root-system of the plant was studied mainly in young individuals, and 

 the penetration of the roots of the fully developed plant was not learned. 

 A young plant of Mentzelia with a shoot 2.3 cm. in length had a tap root 

 1 1 .5 cm. long. The laterals in young plants are numerous, unbranched and 

 occur singly. In older plants the laterals are in groups, as in Malva and 

 other annuals. 



MiCROSERIS TiNEARIFOLIA. 



Microseris is abundant on Tumamoc Hill and on the slopes of the bajada 

 to the north. The specimens examined were growing near the Desert 

 Laboratory on Tumamoc Hill. The root-system, as shown in plate 6, is 

 characterized by the prominence of the tap root and by the slenderness of 

 the laterals of the first order. The root-system of a mature plant, with a 

 shoot 20 cm. in length, consisted of a tap root, 2 mm. in diameter at the 

 crown and over 11 cm. long, and a few filamentous laterals which were 

 borne for the most part within 5 cm. of the surface of the ground. The 

 laterals branched sparingly. Microseris is among the most deeply rooted 

 of the winter annuals. 



MoNOLEPis Chenopodioides. 



Monolepis chenopodioides was collected for study in February from the 

 immediate vicinity of the Desert Laboratory where it was growing in 

 abundance. Its tap root, as plate 7 shows, is the most prominent feature 

 of its root-system, although laterals are early developed. The laterals of 

 the first order, although small as compared with the tap root, are, however, 

 rather coarse, and branch but little. Thus the root-system of Monolepis 

 should be classified as extensive. 



Orthocarpus Purpurascens. 



Orthocarpus occurs on the upper slopes of the baia 'a especially, and 

 only appears in abundance if, as happened in the spring of 1908, when the 

 root-system of the annual was studied, the rains have been copious. It is 

 a low annual, seldom above 15 cm. in height, of a gray-green color, and has 

 lately been found to be an habitual root parasite.* 



*Cannon: The Parasitism of Orthocarpus purpurascms Benth., Plant World, vol. 11, 

 1909, p. 259. 



