C. E. Beecher — Origin and Significance of Spines. 317 



terminal leaflets into spines (figures 71, 72). In Mach cerium 

 the stipules are converted into thorns. 62 A tropical Bigonia 

 (B. argyro-violacea) has normal full-sized simple leaves, and 

 suppressed leaves bearing two opposite leaflets on one stalk, 

 and ending "in a structure which divides into three limbs, 

 with pointed hooked claws, and which is not unlike the foot of 

 a bird of prey." 38 



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 sec- 

 ondary and not a direct result of suppression. Bailey 2 asserts 

 that, "probably the greater number of spinous processes will 

 be found to be the residua following the contraction of the 

 plant body." This connection is very apparent in the consid- 

 eration 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 explanation. 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 (D 3 ), and enable the plant to 

 cling to a support, but these utilitarian properties cannot be 

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

 bly 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 nor- 

 mal plant structures, and to the general suppression of the 

 plant body as affording a more primary conception of the 

 causes and modes of spine growth among climbing plants. 



In many cases of retrogressive series of animals, there seems 

 to be a close parallelism with some of the characters observed 

 among the climbing plants. If the Ammonite family during 

 the Cretaceous, or near the close of the Mesozoic, is taken as 

 an example, it cannot be said that the environment of these old 



