66 Rcxrr habits of desert plants. 



The shoot of the larger specimen of Franscria from the bajada was 50 

 cm. high and was composed of numerous slender branches. The species 

 is well covered with leaves, so that, compared with most of the local shrubs, 

 it has a large transpiration surface. The leading characters of the shoot 

 are shown, but rather imperfectly, in plate 17. 



The root-system consisted of a tap root bearing several laterals and of 

 numerous adventitious roots originating from the bases of the shoot. The 

 tap root went straight down 29 cm., where it forked, one root penetrating the 

 rotten caliche to a depth of 38 cm. from the surface of the ground. About 

 12 cm. from the surface, the main root gave off several laterals which 

 reached as far as 1.6 meters from the main axis and lay 12 to 30 cm. deep. 

 Thus the roots are confined to soil above the hard caliche. The roots are 

 dark brown, very brittle, and are ridged longitudinally with cork. They 

 are always slender; the largest root, the tap root, 2 cm. from the crown, 

 was only 8 mm. in diameter. 



The younger specimen of Franseria studied was 3 meters distant. Its 

 shoot was 14 cm. high and composed of four separate branches. Upon dig- 

 ging, it was learned that the surface soil was relatively deep, and that over 

 a small ar£a the hard caliche had given place to adobe, a condition similar 

 to that shown in plate i. 



A main root of the young plant was traced down i meter, and evidently 

 penetrated farther. The main root gave off one lateral 3 cm. long, another 

 less than 10 cm. long, and a third which, shortly after leaving the main 

 root, penetrated to a depth of 60 cm. The vertical extension of the root- 

 system is shown in fig. 9. No roots of neighboring plants were seen in the 

 root-areas of this plant. 



For comparative purposes, the roots of a Franscria growing on the flood- 

 plain not far to the east of the proper habitat of the species were also 

 examined. The soil is a sandy loam, sand and adobe, of a depth exceeding 

 2 meters. No excavations have been made below this level. The place is 

 occasionally flooded and the water-table is variable as to its depth, but 

 probably the ground at a depth reached by roots is moist all of the year. 



The leading woody plants of the flood-plain in the immediate neighbor- 

 hood of the specimen of Franseria studied were Acacia greggii, Covillea 

 tridentata, Ephedra trifurca, Lyciion andersonii, Parkinsonia torreyana, and 

 Prosopis velutina. The flood-plain is the characteristic habitat of all of 

 these except Covillea and Franseria. 



The leading points of interest in the root-system of Franseria from the 

 flood-plain were as follows: A tap root which went straight dow^n to a depth 

 of 1.8 meters w^as only 5 mm. in diameter at its crown. For the most part 

 the laterals arose within 15 cm. of the surface, and one of these extended 

 1.4 meters in a fairly horizontal direction (see fig. 9) ; another took a doAvn- 

 ward course at an angle approximating 45° until a depth of 45 cm. was 

 reached, after which it was approximately horizontal; a third root, after 



