194 BOTANICAL GAZETTE [SEPTEMBER 
but the height of the plant above the nearest arroyo. Even though 
there is a stream in the arroyo only a few hours each year, there is 
probably a layer of moister soil on a level with the bottom of the 
arroyo, on account of a slow creep of ground water toward the arroyo 
and the conservation of the moisture by the dry sand covering it. 
Dysodia acerosum.— 
The habitat of this plant 
is the same as that of 
Ephedra. The aerial 
parts of the plant form 
a compact, much- 
branched tuft. The 
leaves are needle-like. 
The individual photo- 
graphed grew in sandy 
adobe with large pebbles 
(fig. 21). The plant 
had a stout tap root 
with a few large laterals 
arising close together a 
short distance below the 
surface of the soil. The 
tap root shown meas- 
ures 39 in., but it was 
probably several inches 
aie longer. Both tap and 
Fic. 23.—Root system of Opuntia fragilis lateral roots bore 
numerous fine branches. 
This species probably has the largest root system in proportion to 
the size of the aerial parts of any of the plants of the region. 
Chrysothamnus Bigelovii—This is the dominant plant in an 
association characteristic of the lower parts of the sides of arroyos. 
The principal photosynthetic work is done by the almost leafless 
green stems. The root system is of the ne type. Fig. 22 
shows only a part of the root system of a rath Later 
excavation showed laterals up to go and 100 in. ». in length and a tap : 
- root about roo in. ae eee te 
