1917} MARKLE—ROOT SYSTEMS 203 
is very prominent. A comparable adjustment of the roots of 
plants with reference to soil moisture probably exists in all types 
of associations, but especially in arid habitats soil moisture is a 
limiting factor and root competition is more severe. Observations 
of the distribution of the roots of some of the associations were made 
on rapidly eroding arroyo banks and sand and gravel pits. 
In the Bouteloua association, the most superficial layer of roots 
is that formed by the grasses, principal among which are Bouteloua 
ertopoda and Hilaria Jamesii. Most of the roots of these grasses 
occupy the upper 2 in. of soil, although some of them go much 
deeper. The roots of Hilaria are very tough and woody and reach 
a length of 6 ft. or more. The thorough permeation of the upper 
Fics. vit ss Fe 3% horizontal extension of root System of rie canescens 
at depth of 
ele A eae a aN 
for the relatively pure growth of grasses in this association. They 
so thoroughly remove the water from the superficial layer that 
seedlings of deeper rooted Plants perish before the lower, moister a 
_ layers are penetrated. Over larg ae 
Sg ke aS eT BEI 
: ruderals ane £7..85 
ih the Dysodia-E phedra association, th 
