66 



TRANSFORMATIONS OF THE LEAF. 



314. The petiole of the leaf of Clematis, otherwise unclianged, 

 coils like a tendril for the support of the vine. In the greenbriar, the 

 stipules are changed to tendrils, which thus arise in pairs from the base 

 of the petioles. So probably in the gourd tribe. 



316. But the tendrils of the grape-vine are of ,i different nature. From 

 their position opposite the leaves, and tho tubercles occasionally seen upon them, 

 representing flower buds, they are inferred to be abortive, or transformed flower- 

 gtalks. 



179 180 ISl 183 



Thorns. 179, Cratajgus parvifolia (thorns axillary.) 180, Iloney-locnst. ISl, Common locust 

 182, Borboris, a, a, its thorns. 



316. Spines. Many plants arc armed, as if for self-defense, with 

 hard, sharp-pointed, woody processes, called spines or thorns. Those 

 which are properly called spines originate from loaves. In Berberis 

 the spines are evidently transformed lea\'cs, as the same plant exhibits 

 leaves in every stage of tho metaraorpiiosis. In goat's-lhorn (Astraga- 

 lus tragacanthiis) of S. Europe, the petioles change to spines after the 

 leaflets fall off. In the locust (Robinia), there is a pair of spines at the 

 base of the petiole, in place of stipules. 



31'if. Thorns originate from axillary buds, and are abortive branches. This is 

 evident from their position in the hawthorn and Osage orange. Tho ajiple and pear 

 tree in their wild state produce thorns, but by cultivation become thornless 

 that is, the axillary buds, through better tillage, develop branches instead of thorns. 

 The terrible branching thorns of the honey-locust originate just above the axil, from 

 accessory buds. 



318. Prickles differ from either spines or thorns, growing from tho epidermis 

 upon stems or leaves, at no determinate point, and consisting of hardened cellular 

 tissue, as in the rose, bramble. 



319. Bracts. By a more gentle transformation, leaves pass into 

 bracts, which are those smaller, reduced loaf-forms situated near and 

 among the flowers. So gradual is the transition from loaves to bracts 



