Ch. Ill, 9; SPECIAL FUXCTIOXiS 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 

 \iew that they repre.-ent 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, 

 a.s is believed true in the 

 Cactuses 'Fig. 54 j : in 

 others they represent the 

 midrib and two lateral ribs 

 of a leaf, as in Barberry 

 ("Fig. bb) ; in Euphorbias, 

 when paired, they clearly 

 represent stipules (Fig. 

 bl ) ; while in some tropical 

 climbers the stipular spines 

 are very strong do'^Ti-n-ard- 

 turned hooks which catch 

 firmly upon other vegeta- 

 tion. 



AMiile the lilade is the 

 chstinctive chlorench\-ma-carr},-ing 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 



Fig. oU. — .Stages in the twiidng of a 

 tendril, of Bryonia; X -{■ This is a 

 stem tendril, but the method is the 

 same in leaf tendrils. (Drawn, ^ith 

 slight alterations, from a wall-chart by 

 Errera and Laurent.; 



