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 8 

 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 



