DEFENCE. 107 



it is often found that the tips of branches, or of leaves 

 or leaf-teeth become dead, and remain as sharp spines or 

 thorns, which are found exceedingly useful to the plant 

 in keeping off the camel, the goat, or other grazing 

 animal. Such thorns and spines are very abundant 

 in open scrub-like country, such as the Karoo of 

 Africa, and the deserts of Central Australia, where a 

 grass, " Spinifex," prevents thousands of acres from being 

 cultivated. It is in the deserts, the natural home of the 

 camel, the horse, and the ass, that thorns are most 

 abundant. Almost every plant in such regions is pro- 



FiG. II. — Spines of the involucre bracts of Centaurea calcitrapa. 



tected either by thorns or spines ; or by strong balsams 

 or poisonous secretions. Amongst the most curious 

 examples are the following : Carlina acaulis, in which the 

 little rosette of leaves lies absolutely flat on the sand, 

 whilst in the centre, exactly in the position to pierce 

 the nostrils of a grazing animal, there is a circle of 

 sharp spines, each of which is from i to 2 inches in 

 length. Those of Centmirea calcitrapa are also 

 modified bracts of the involucre. The Gum Tragacanth 

 {^Astragalus tragacanthus) has also a rosette of 

 leaves which are pinnate, and consist of some 

 7-8 pairs of leaflets. These leaflets soon drop 



