ROOT HABITS OF DESERT PLANTS. 27 



The main characters of the root-system are the relatively large size of the 

 laterals of the first order and the paucity of filamentous roots. The ulti- 

 mate roots are of two sorts, a longer and a shorter kind. The latter are 

 rudiments and are borne in the axils of the former; they are shown, but 

 imperfectly so, in plate 3. The plant thus shows a tendency to produce 

 roots in groups. 



Cladothrix Lanuginosa. 



The specimens of Cladothrix studied were removed from the soil on August 

 19, and were mature plants with large leaves and shoots about 19 cm. in 

 length. The main root was over 2 mm. in diameter at the crown and was 

 6 cm. in length with the tip atrophied, but it was continued 1 7 cm. farther by 

 two successive laterals, so that the depth attained was over 23 cm. The 

 laterals are numerous and rather fine and for the most part arise within 4 

 cm. of the surface of the ground. They bear laterals of the second order, 

 which, in turn, are branched. The ultimate roots are represented in very 

 numerous cases by rudiments. 



As a whole, the root-system of Cladothrix is characterized by the slender- 

 ness of its roots, a large number of which are filamentous. Root hairs are 

 present in great abundance. The absorption faculty, therefore, is a feature 

 strongly developed. 



Datura sp. 



The specimen of Datura studied was collected on August 10 and was 

 immature. The shoot was 15 cm. in length and bore several large leaves. 

 The root-system is characterized by the very large number of fine roots, 

 which are usually long. At the crown the main root is 4.5 mm. in diameter, 

 but tapers rapidly and is under 10 cm. in length. At a point 8 cm. beneath 

 the surface of the earth it was less than 0.5 mm. in diameter. The laterals 

 are all long and of two diameters, a few are noticeably heavy, and man}^ 

 are very slender. All of the laterals of the first order bear filamentous 

 branches which are long and branch but little. The roots of the third order 

 are especially fine. Plate 3 shows only imperfectly the density of the 

 absorption system of Datura. 



Ditaxis Humilis. 



The species of Ditaxis examined is a perennial herb, which was in llower 

 when collected, August 10. The shoot is leafy and over 12 cm. long. The 

 roots reach deeper than those of the other herbaceous forms studied and 

 the absorption system is poorly developed. The main root is relatively 

 slender; it is over 23 cm. in length and is only 2.5 mm. in diameter at the 

 crown. Four large and numerous small laterals of the first order arise 

 within 8 cm. of the surface of the ground. Very few laterals are given off 

 from the main root below this. The laterals may extend 1 1 cm. and over, 

 and mostly are in groups of three or four. The laterals of the second order 

 are filamentous and rarelv branch. There are no rudimentarv roots. 



