56 ROOT HABITS OF DESERT PLANTS. 



Of the specimens of sahuaro studied, the first to be described was a healthy 

 and very vigorous plant i . 2 meters high and 35 cm. in diameter at the largest 

 part. The anchoring system consisted of a stout tap root which went 

 straight down to a depth of 30 cm., or to the caliche, and a few laterals, of 

 which one ran horizontally about 25 cm. The superficial system arose from 

 the main root close to the surface of the ground and was composed of about 

 6 main laterals which radiated from the main axis so as to divide fairly 

 equally the available area between them. They extended from 1.5 to 5 

 meters from the central axis, and branched but little. As an instance of 

 the latter characteristic it may be stated that four of the laterals did not 

 give off branches during 1.5 meters of their course, and one root, which 

 branched freely at the tip and was 3 meters long, bore only 4 branches. The 

 superficial roots end in a tuft of fine rootlets, nearly all of which die with 

 the advent of the drier season, and groups of delicate rootlets are borne by 

 the superficial roots the greater part of their length. It is probable that 

 these fine rootlets are produced each year with the coming on of favorable 

 conditions of growth and water absorption, as in Franseria dcltoidca and 

 other desert plants. The plant is thus enabled to exploit thoroughly more 

 of the root-area than would otherwise be possible. 



The superficial portions of the root-system penetrated the ground to a 

 depth which was fairly uniform, although in no case very great. For 

 example, one root leftthemain root at the surface of the ground, and bothit 

 and its three main branches, with little deviation, ran 7 cm. from the surface. 

 Another root ran from 3 to 40 cm. from the surface, but gave off a branch 

 which went straight down to a depth of 1 7 cm. Others of the superficial 

 system varied between 5 and 15 cm. in depth, and one dipped under a large 

 boulder, whose lower surface was 30 cm. from the general ground-level, after 

 w^hich it ascended to about the depth characteristic of the older parts. 



Where the larger of the superficial roots left the tap root they were rela- 

 tively heavy, but they tapered rapidly until a small diameter was reached 

 which was maintained with little change for a long distance. A few meas- 

 urements will show the point. One of the larger laterals was 1.6 cm. in 

 diameter at its base; another was 7 mm. in diameter; another root, 45 cm. 

 from its base, was i cm. in diameter, and it extended 2.5 meters beyond the 

 point of measurement. Roots not above 50 cm. long were about 3 mm. in 

 diameter at the base. A root 5.5. meters in length was 2 cm. in diameter 

 30 cm. from its base. 



In a sahuaro of the size of the one under discussion, it is probable, there- 

 fore, that the tap root constitutes the main support by which the plant 

 successfully withstands the pressure of the wind, which at times is heavy, 

 and that the superficial roots function mainly as absorbing organs. With 

 the growth of the plant, however, the need of anchorage is greatly increased 

 and the tap root no longer suffices as the sole stay, so that the laterals assume 

 the added role of mechanical supports and become much changed in size, 

 perhaps in other regards also, to meet the new demands. This develop- 



