Deserts 
Anything that causes a loss of water must surely tend 
to make the tissues more woody and less soft. One of 
the most interesting spiny plants is the gum-tragacanth 
(Astragalus). This has compound (pinnate) leaves, which 
are at first quite green and not perceptibly different from 
ordinary leaves. But as the leaf grows older it dries up 
and the leaflets fall off,so that in the end the leaf-stalk and 
midrib becomes a sharp, hard spine, which is an efficient 
protection to the next season’s green and juicy foliage. 
The history of a hedge-thorn is quite similar. In its 
extreme youth it is quite soft and flaccid except for the 
extreme tip, which may have been killed by frost or ex- 
posure to sunlight. If the hedge is pruned or much cut 
about, there will be a great loss of water from the scars 
and cuts made by the bill-hook, and in such a case an 
unusual number of the branches become sharp, hard, and 
business-like thorns (see also p. 25). 
It has also been shown by direct experiment that 
thorny plants cease to form these weapons when culti- 
vated in a moist and humid atmosphere. This has been 
done by several observers, and lately by Mr. Cockayne ® 
for Discaria Toumatou. He found that in strong sun- 
light this plant has no leaves, and is very much branched 
and thorny. But in moderate light, and when grown 
with sufficient moisture, it develops leaves and loses 
almost all its thorns. 
As to the common holly, it is the case that its lower 
leaves are often spiny when the upper leaves are per- 
fectly smooth and harmless. We cannot suppose that it 
has the intelligence to know that only its lower leaves 
will be attacked by animals. Has it, like pruned hedges, 
got into the habit of arming its lower leaves in response 
to continual loss of water by evaporation at that level ? * 
Thorns and spines are generally very well placed and 
* «The Romance of Plant Life.” 
176 
