90 STUDIES IN EVOLUTION 



"in a structure which divides into three limbs, with pointed 

 hooked claws, and which is not unlike the foot of a bird 

 of prey." ^^ 



By far the greater number of spines on climbing plants are 

 of the nature of prickles, and are not produced by the sup- 

 pression of any particular organ or organs, but appear usually 

 without any very definite order. They represent outgrowths 

 of the superficial layers, and hypertrophied plant hairs, or 

 trichomes. The cause of these cortical eruptions is not clear, 

 although they seem to be intimately connected with the gen- 

 eral suppression of the plant body. They are therefore a 

 secondary and not a direct result of suppression. Bailey ^ 

 asserts that "probably the greater number of spinous pro- 

 cesses will be found to be the residua following the contraction 

 of the plant body." This connection is very apparent in the 

 consideration of the suppression or contraction of various 

 plant organs, but is less obvious when applied to the surface 

 of the whole plant, though doubtless it is the true explana- 

 tion. In continuation of this idea it may be suggested that 

 the prickles represent aborted attempts on the part of the 

 plant, through hereditary influences, to recover its former 

 normal proportions. Or they may exhibit the action of the 

 law of repetition acting in an organism where the initial 

 cause of spine production is the intrinsic suppression of such 

 structures as leaves, petioles, stipules, etc. The subsequent 

 repetition of spines on other parts of the organism results in 

 a series of homoplastic spines which are not homologous with 

 those first formed. 



The prickles on climbing plants and brambles may often 

 serve for purposes of protection (D3), and enable the plant to 

 cling to a support, but these utilitarian properties cannot be 

 considered as an initial cause. Natural selection, also, prob- 

 ably has fostered the development of certain types of spiny 

 climbers and the production of adaptive characters. Never- 

 theless, in studying these forms, it is necessary to revert to 

 the original consideration of the localized suppression of 

 normal plant structures, and to the general suppression of 



