998 



THE NATURE BOOK 



CACTUS THORN. 



many remarkable devices have been called 

 into being. One of the most widespread 

 contrivances for the defence of the plant 

 is the thorn. These spines are very 

 prevalent among many genera, and in 

 the different species represent modifica- 

 tions of both leaves and stems. The 

 development of the thorn is seen to a 

 remarkable extent in the case of the 

 Cacti, plants which in some ways are 

 more perfectly protected than any others 

 in the world. With many species, 

 particularly among the groups Echino- 

 cactus and Melocactus, the whole plant 

 is covered with spines, which interlace 

 in such a way that no part is left un- 

 shielded. A kind of Prickly Pear, well 

 named Opimtia jormidabilis, produces 

 thorns which are sometimes as much as a 

 foot in length. The desert in which 

 these plants grow is peopled with 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. 



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 exceedingly interest- 

 ing 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 any spines. Now 

 and again both these varieties may be 

 found growing in the same district, some- 

 times quite close to one another. It will 

 be seen, however, that the thornless 

 specimen has been more fortunate in its 

 position than the one which is well clad 

 with its defensive armament. In the 

 former instance the little plant has 

 lighted upon a patch of moist, rich soil ; 

 whilst in the latter case a hard struggle 

 for existence, in a dry situation, has been 

 the fate of the subject. It is very com- 

 monly to be seen that a race of one- 

 time thorny plants will, under cultivation, 

 tend to get away more and more from 

 their spiny character. As domestic 

 subjects, the plants, removed from the 

 fierce fight in the natural world, are no 

 longer in need of protective devices. 



Although in many cases serving the 

 same purpose, the prickles often seen 

 upon the leaves and stems of plants are 

 on different lines from thorns, seeing that 

 they are merely excrescences of the cuticle. 

 The hooked prickles of the Briar Rose peel 

 off with the skin and are quite unattached 

 to the woody portion of the stem. In 

 the Holly, the prickles are to be found 

 springing from the margin of the leaves. 

 It is curious that it is only wliilst the 

 Holly is in its shrubby form that the 

 leaves evidence their prickly nature. As 

 soon as the plant attains to the dignity 

 of a tree the foliage is produced with its 

 margins quite unbroken. In many large 

 bushes, it is possible to trace the process 

 by which the prickles have been gradually 

 discarded ; and from a single shrub an 

 interesting series of leaves may be 



