1917] MARKLE—ROOT SYSTEMS 201 
eleagnifolium, shown in fig. 11. A layer of soil difficult of penetra- 
tion may cause much distortion of roots entering it, as seen in the 
diagram illustrating the roots of Atriplex canescens. 
The most striking instance of the effect of a variation in the 
water content of the soil is shown in the roots of plants growing 
along arroyos. Nearly all of these are characterized by long tap 
roots, the length of which apparently is determined by the height 
of the base of the plant above the moister soil below the level of the 
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