334 G. E. Beecher — Origin and Significance of Spines. 



namely, either to their complete disappearance or to their par- 

 tial suppression into hard spiniform processes or thorns. Thus, 

 both leaves, branches, and other parts of the plants may become 

 reduced to their axial elements, bringing about what is com- 

 monly termed spinescence. 



The spiny character of these plants is therefore one of the 

 results of an arid environment, and it may or may not be of 

 sufficient frequency to give an especial character to a particular 

 desert flora. There is, moreover, a secondary influence which 

 has an effect in determining the abundance of spinose plants in 

 desert as well as in many other situations. This relates to the 

 destruction of the edible unarmed species by herbivorous ani- 

 mals, and the comparative immunity of the spiny types. Thus, 

 in old pastures, the prevailing flora is apt to be one that is 

 offensive to grazing animals. This character is generally given 

 by poisonous plants or those having a disagreeable flavor, or by 

 those whose form or spiny structures afford protection. 



This secondary influence by grazing animals may have had 

 some effect in determining the particular abundance of spiny 

 plants in certain desert regions, and their comparative infre- 

 quency in other similar regions. In either case, the unfavor- 

 able environment brings about a suppression of structures, and 

 one type of this action results in the production of spines. 

 These represent the limits of retrogression before the part 

 becomes entirely obsolete. 



Wallace has criticised Henslow's views on the origin of 

 xerophilous plants and their distribution. It is believed that 

 the views here offered remove some of the objections, and 

 bring the opinions of these authors into greater accord. 



Under arid conditions, bracts, stipules, leaves, and even 

 branches may become spinescent. Some forms in which the 

 spinose character has not as yet become permanently fixed by 

 heredity, when transported or found living in moister and 

 richer soils, develop normal leaves or branches, and lose their 

 spinescence ; others, like the Cactus, retain their spines under 

 similar changes ; while still others, as Acanthosicyos horrida 33 

 cannot be artificially cultivated, and have become truly xlero- 

 philous types. 



As examples of plants which lose their spines by cultivation, 

 the Pear, species of Rose, Plum, etc. (Henslow), may be cited. 

 According to Henslow, 33 others, as Onomis spinosa, have an 

 especially spiny variety {horrida) living on sandy sea-shores, 

 while in more favorable natural situations, the same plant 

 becomes much less spiny, and under cultivation loses its spines. 

 M. Lothelier 42 also found that by growing the Barberry (Ber- 

 heris vulgaris) in moist air, the spines disappeared, the paren- 

 chyma of the leaves being well formed between the ribs and 



