C. E. Beecher — Origin and Significance of Spines. 335 



vems. 



Dry atmosphere and intense light both favored the 

 production of spines. 



Henslow 33 cites the genus Zilla as a desert plant in which the 

 branches are transformed into spines, Echinojps for a similar 

 modification of the foliage, Fagonia for spiniform stipules, 

 and Centaurea for spinescent bracts. As further illustrations 

 taken not only from desert plants but also from others com- 

 monly found in dry, rocky, or unfertile situations, the follow- 

 ing 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 Cactacese of America, the columnar Euphorbiacese of 

 Africa and southern Asia, and are also familiar in the half- 

 shrubby Tragacanth bushes (figures 55, 56) 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 Robinia, of which the Common Locust 

 {Robinia pseudacacia) furnishes a well-known illustration 

 (figure 57). Spiniform bracts are best known among the 

 Tfiistles (Cirsium lanceolatum, C. horridulu?n, etc.). 



54. 



55. 



56. 



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

 into spines. (After Kerner.) 



Figure 55. A single leaf of Tragacanth (Astragalus Tragacantha) 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 

 generally 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 primary 

 factor, its influences are mainly exhibited through secondary 

 or resultant conditions. In some cases, however, it is possible 



