ROOT SYSTEMS 29 



is that of Weaver (19 19, 1920), on the plants of the prairies 

 and chaparral of Nebraska, the prairies of Washington and 

 Idaho, the plains, sandhills, gravel-slides, and forests of 

 Colorado. Beside the plant to be studied a trench was 

 dug and the whole root system was carefully excavated, 

 charted, and photographed in its finest details. Weaver 

 has charted the systems of about two hundred plants. 

 Besides giving an accurate picture of the architecture and 

 extent of the root system, his work is concerned with the 

 relation of definite types to their special habitats, and also 

 with variations in the root system of a single species under 

 different conditions. 



The root systems of the prairie plants of Nebraska are 

 characterised by their very great depth. Of forty-three 

 perennials investigated only six, all grasses, have root 

 systems less than 3*5 ft. deep ; such are Elymus canadensis 

 (i'8 ft., 1*3 ft.), Koeleria cristata (i"8 ft., i"3 ft.), and Stipa 

 spartea {z'z ft., 1*5 ft.). Nine have roots reaching depths 

 of 3*5 to 6 ft. ; such are Andropogon scopariiis (5"4 ft., 

 3*2 ft.), Solidago rigida (5"2 ft., 3*3 ft.), Verbena stricta 

 (4*3 ft., 37 ft.). The roots of the remaining twenty-eight 

 species reach greater depths than 6 ft., most about 10 ft., 

 and several from 17 to 20 ft. ; examples of these are Agro- 

 pyrum repens (8 ft., 6 ft.), Aster multiflonis (8 ft., 5 ft.), 

 Ceanothus ovatiis (i4'5 ft., 11 ft.), Rosa arkansana (zi'Z ft., 

 16 ft.), Solidago canadensis (11 ft., 8 ft.). Of greater 

 importance than the maximum depth is the working depth , 

 by which Weaver means " the average depth reached by a 

 large number of roots or branches of the root system, and to 

 which depth considerable absorption must take place. It 

 has no absolute value, like maximum depth of penetration, 

 but can usually be determined for most root systems with a 

 considerable degree of accuracy." The working depth of 

 the first group ranges from 1*2 to i'5 ft., of the second 

 from I "7 to 3*8 ft., and of the third from 3*3 to 16 ft. The 

 second figures quoted for the examples given above are the 

 working depths. An example of a plant of the third group, 

 Liatris punctata, is shown in Fig. i. 



