THE PLANT AND ITS ENEMIES 87 



Melocactus, the whole plant is covered with spines, 

 which interlace in such a wa}' that no part is left 

 unshielded. A kind of Prickly Pear, well named 

 Opuntia formidabilis, produces thorns which are 

 sometimes as much as a foot in length. The 

 desert in which these plants grow contains but 

 a comparatively sparse vegetation, and the juicy 

 stems of the plants would be peculiarly liable 

 to be attacked by hungry and thirsty animals. 

 Indeed, even with its armament of thorns, the 

 Cactus is not absolutely secure. It is said that 

 the Mexican ponies have learnt the art of kicking 

 away the spines with their heels, and then slaking 

 their thirst at the bleeding wounds inflicted on 

 the plant. 



It is a very significant circumstance that the 

 production of spines seems to take place to the 

 greatest extent when the plant is growing under 

 adverse conditions. In this connection an exceed- 

 ingly interesting species is the Rest Harrow 

 (Ononis arvensis). As is well known, this plant 

 is to be found fairly common in two varieties, 

 called respectively 0. spinosa and O. inermis. 

 As is implied by the name, one plant is thorny 

 and the other is quite devoid of spines. Now 

 and again both these varieties may be found 

 growing in the same district, sometimes quite 



