ORIGIN AND SIGNIFICANCE OF SPINES 



others commonly found in dry, rocky, or unfertile situations, 

 the following examples may be taken, some of which are 

 familiar cultivated species: The stunting of branches into 

 spines is common among neglected Pear and Plum trees, 

 and is a normal character in the Hawthorn, Honey-locust, 

 Cytisus (figure 54), Vella^ etc. Leaves transformed into 

 spines are characteristic of the Cactacea3 of America, the 

 colunniar Euphorbiacese of Africa and southern Asia, and 

 are also familiar in the half-shrubby Tragacanth bushes 

 (figures 55, bQ') so common in southern Europe, especially in 

 the eastern portion, and in the ordinary Barberry (figure 13). 

 Spiniform stipules are usually present in the species of 

 Mobinia, of which the Common Locust (^Rohinia Pseudacaeiii) 

 furnishes a well-known illustration (figure 57). Spiniform 

 bracts are best known among the Thistles QCirsium lanceo- 

 latum, C. horridulum, etc.). 



54 



55 



56 



57 



J 



Figure 54. — The spiny Cytisus (C. spinosus) ; showing suppression of branches 

 into spines. (After Kerner.) 



Figure 53. — A single leaf of Tragacanth {Astragalus Tragacantka) , from 

 which the three upper leaflets have fallen. (After Kerner.) 



Figure 56. — Leaf axis of the same, from which all the leaflets have fallen. 

 (After Kerner.) 



Figure 57. — Twig of Common Locust (Robinia Pseudacacia) ; showing spines 

 representing stipules. 



As the restraint of an environment acting on an animal 

 so generall}' results in the disuse and atrophy of the organs 

 affected, most cases will have to be considered under the 

 head of disuse. Therefore, while the environment is the 



