55 
resistant to grazing, provided there is adequate soil 
moisture. 
Glycine falcata also has a deep tap root. In addi- 
tion, there are wiry rhizomes which spread horizontally 
1 to 3 inches below the surface (Pig. I.C.). At intervals 
they give rise to new vegetative shoots. Within the soil 
at the base of some of these shoots there develops a soli- 
tary flower, followed by a single-seeded, indehiseent pod. 
The seeds in these pods are larger than those in the 
ordinary pods which are borne in a raceme on the 
aerial shoots. However, the subterranean seeds are viable 
and give rise to plants which appear in no way different 
from those grown from " normal” seeds. The purpose of 
these subterranean pods is a matter for speculation, but 
they are well placed to ensure the survival of the species 
iu a region where the drying influence of the environ- 
ment is very great and where ants carry away many 
seeds shed on the surface of the ground. 
Mulgu ( Acacia uncura) differs markedly from the 
three examples so far quoted. Whereas the others are 
herbaceous, Mltlga is a tree. The soils it grows in, too, 
are markedly different. Mulga soils are usually red 
brown loams or fine sandy loams with a hard pan be- 
neath them. The nature and depth of the hard pan 
varies from place to place, but it serves to retard the 
downward movement of soil water, and above it there 
is a zone of accumulation. The root system of mulga 
(Fig. 1.1).) has differentiated in much the same way as 
that of Mitchell Grass. In the surface soil there are 
numerous shallow roots, much branched and spreading 
widely around the tree. Below that, a tap root goes 
down nnbranchcd to about 12 inches above the hard pan. 
There, the roots divide and ramify through the soil 
overlying the pan and are favourably situated to utilize 
the water stored in the soil after soaking rain. 
Many other plants in the semiarid region have root 
systems similarly modified to utilize the erratic rainfall 
to the best advantage. 
