Ch. Ill, 9] SPECIAL FUNCTIONS OF LEAVES 



79 



Leaves are also often modified to spines, especially in 

 plants of dry places. The significance of spines, however, is 

 uncertain; for the older 

 view that they represent a 

 protection against animal 

 enemies seems inadequate, 

 while the newer idea that 

 they result from a struc- 

 tural degeneration of leaves 

 rendered superfluous by 

 changed habit has not won 

 acceptance. In the trans- 

 formation they lose their 

 chlorophyll and flat form, 

 and become slender, coni- 

 cal, and hard. In some 

 cases each spine represents 

 a single transformed leaf, 

 as is believed true in the 

 Cactuses (Fig. 54) ; in 

 others they represent the 

 midrib and two lateral ribs 

 of a leaf, as in Barberry 

 (Fig. 55) ; in Euphorbias, 

 when paired, they clearly 

 represent stipules (Fig. 

 57) ; while in some tropical 

 climbers the stipular spines 

 are very strong downward- F J G - 52 —stages in the twining G f a 



. , , tendril, of Bryonia ; X \. This is a 



turned hooks which catch 8te m tendril, but the method is the 



firmly upon Other vegeta- ««* in leaf . tendrils. (Drawn, with 



slight alterations, from a wall-chart by 

 tion. Errera and Laurent.) 



While the blade is the 

 distinctive chlorenchyma-carrying part of the leaf, the 

 foliage function is in some cases assumed by petioles or 

 stipules, the blade being more or less suppressed. Thus, in 



