66 



TRANSFORMATIONS OF THE LEAF. 



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

 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. 



315. But the tendrils of tub grape-vine are of a different nature. From 

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

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

 B talks, 



1"9 180 131 182 



Thorns. 179, Crataegus parvifolia (thorns axillary.) ISO, Honey-locust. 181, Common locust 

 182. Bcrberis, a, a, its thorns. 



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

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

 which are properly called spines originate from leaves. In Berberis 

 the spines are evidently transformed leaves, as the same plant exhibits 

 leaves in every stage of the metamorphosis. In goat's-thorn (Astraga- 

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

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

 base of the petiole, in place of stipules. 



317. Thorns originate from axillary buds, and are abortive branches. This i3 

 evident from their position in tho hawthorn and Osage orange. The apple and pear 

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

 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 

 Qccesswy buds. 



318. Prickles differ from either spines or thorns, growing from the 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 leaf-forms situated near and 

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



