53 
considerably more than this is shaded by the culms. The 
interstices between tlic tussocks are occupied by other 
species, either annuals with shallow root systems and of 
snort life span, or other perennials. 
The root system of Astrebla is interesting. Figure 
l.A. illustrates diagranunatieally a tussock 6 inches in 
diameter at ground level growing in a dry soil. From 
the rhizomes, wiry roots arise. They spread obliquely 
outwards and downwards through the loose surface layer 
of soil and may reach a diameter of 36 inches at 6-9 
inches below the soil surface. In this zone numerous 
small lateral roots are present. At the base of the upper 
lionzon the roots turn sharply and continue downwards 
unbranched through the massive clay until they reach 
me transition zone. Here they branch again, ramifying 
through the transition zone and extending into the yellow 
brown clay beneath. Most of the fine roots lie between 
30 and 48 inches below the soil surface, and a few extend 
deeper than that. 
Obviously the surface roots are in a position to tap 
moisture from small showers which is absorbed by the 
surface soil but which does not penetrate far. In the 
zone of severe fluctuations in soil moisture, the roots are 
vertical and unbrauched. This allows of minimum 
damage by the large horizontal movement which takes 
place in this part of the soil. The great number of small 
loots present below 30 inches allows the plant to utilize 
water stored in the zone of accumulation. 
Psoralen cinerea has a short tap root which goes 
down almost unbranched for about 30 inches where it 
divides and ramifies through the soil (Fig. I.B.). In the 
surface layer, there are a few stout lateral roots, mainly 
horizontal and with numerous branches. The crown from 
which new vegetative shoots arise is just below the sur- 
face. The first shoots are stout, erect and branched 
only towards the top. Bach branch bears a raceme of 
flowers. After the seeds have ripened, the central stem 
dies, but if sufficient soil moisture is still available new 
branches arise from the crown. These branches are 
prostrate or obliquely ascending. Though many of them 
produce flowers, the racemes are neither so long nor so 
vigorous as on the first, erect, shoots. Because of this 
capacity to produce prostrate shoots, Psoralen cinerea is 
