198 BOTANICAL GAZETTE . [SEPTEMBER 
confined almost entirely to the space between the 16 and 24 in. 
depths. The effect of the position of the plant with reference to 
the arroyo bed is shown here. 
Gutierrezia Sarothrae——This semi-evergreen shrub is a common 
ruderal in many associations of mesa, mountain, and valley, but 
especially on the mesa, where grazing has been a greater disturbing 
factor. The plant shown in the photograph grew near the bottom 
of an arroyo and had a root system of the generalized type. The 
Fic. 27.—Yucca glauca 
plant shown in the diagrams was a small specimen 8 in. in height 
and grew near the edge of the steep bank of anarroyo bed. The tap 
root was especially well developed and extended vertically to a 
depth of 44 in., where it reached the level of the bottom of the 
arroyo. Here the tap root turned and extended out under the bed 
of the arroyo a distance of roo in., branching freely. The horizontal 
part of the root was within 2 in. of the surface and bore numerous 
fine absorptive roots. It is evident that the unusual development 
of this root system is a response to moisture conditions, and it 
is doubtless to this ability to respond to varying conditions that 
