ROOT-SVSTEMS (IF TEREXXIALS. 67 



reaching out about 20 cm. from its place of origin, turned directly down- 

 ward to a depth of 78 cm. 



Besides roots arising on the main root, and of primary origin, there were 

 over 50 slender adventitious roots, about 3 mm. in diameter, which arose 

 from the shoot bases and from the crown of the main root. For the most 

 part, these roots took a horizontal direction, but one of them ran downward 

 and was traced to its end at a depth of 1.3 meters. Such laterals as lay 

 about 15 cm. from the surface of the ground bore groups of filamentous 

 roots (plate 17), which were dead when the study was made. The rootlets 

 were 2 cm., more or less, in length and were six or so in a group ; the groups 

 were from 5 mm. to 2 cm. apart. 



Filamentous roots were seen also in the bajada-grown plants, and in 

 others from Tumamoc Hill. In January, 1910, after the soil had been 

 moistened some days by rains, freshly formed rootlets were seen on many 

 roots of Franseria. These roots, therefore, are formed in winter, and 

 probabh- in summer as well, and are very short-lived. Xo study has yet 

 been made of the conditions under which the rootlets are developed, their 

 period of activity, or whether, if conditions remained constantly favorable, 

 they would be active indefinitely. 



Kr.vmeri.v C.\XESCENS. 



One of the most generally distributed species in the vicinity of the 

 Desert Laboratory is Krameria canescens, occurring as it does on each of the 

 physiographic areas treated in the present paper. It attains its largest size 

 where the water relations are most favorable, as on the flood-plain of the 

 wash, on the bajada near it, and on Tumamoc Hill. As has been shown 

 elsewhere* Krameria is an habitual parasite and this is probably the lead- 

 ing cause for the wide distribution noted. 



The first specimen of Krameria, of which the root-system was examined, 

 was an isolated plant on the flood-plain of West Wash. The soil condi- 

 tions of the place have already been noted ; in brief they were : a sandy clay 

 over 2 meters deep and relatively favorable water relations, as evidenced 

 by the occurrence of the most hygrophilous of the native species. 



The specimen studied had a much-branched shoot, about 60 cm. high, 

 which was leafless in February. The general character of the shoot is shown 

 in plate 17. The root-system consisted of a short tap root, no longer living, 

 and six, or more, laterals which extended as far as 2 meters from the main 

 axis (fig. 10). Besides the main laterals there were about an equal number 

 of more slender ones, less than 2 mm. in diameter and about 20 cm. long, 

 which ran in a horizontal direction. The longer roots, from 4 mm. to i cm. 

 in diameter at the base, lay from 13 to 18 cm. beneath the surface. In 

 addition to these roots, several were dead and decaying. A very striking 



*Conditions of Parasitism in Plants, by D. T. MacDougal and W. A. Cannon. Publi- 

 cation No. 129, Carnegie Institution of Washington, loio. 



