Ch. Ill, 9] SPECIAL FUNCTIONS OF LEAVES 



81 



rather a kind of morphological entity easily specialized in 

 diverse directions. Recent investigations have shown that 

 leaves containing stipules receive from 

 the stem three sets of veins, from two of 

 which the stipules are supplied, while 

 leaves lacking stipules receive but one 

 set, or vein. Since the original or primi- 

 tive leaf of our modern trees was appar- 

 ently three-lobed, the stipules may repre- 

 sent the two lateral lobes, which became 

 reduced as the middle lobe developed 

 into the leaf blade of our existent plants. 

 Not all paired structures at the bases 

 of leaves are stipules. In Pereskia, a 



climbing Cactus, the 



paired hooks whereby 



the plant clings to a 



support are the* first two 



spines of an axillary 



cluster, and in some kinds 



of Aristolochia the leaf- 



Fio. 55. 



Leaf spines 

 X *. 



like seeming stipules are °[ fx Ba £berry 



• i + u t * i if t (After Gray,) 

 simply the first leaf of an 



axillary branch. In the Telegraph Plant 

 (Fig. 58), they are leaflets, much smaller 

 than the terminal leaflet ; and in this plant 

 they have further the remarkable property, 

 that, for reasons uncertain, they are con- 

 stantly rising and falling, in short jerky 

 motion suggestive of the arms of the old 

 semaphore telegraph, — whence of course the 

 plant's name. 



Typically, leaves are flat plates of tissue, 

 and in their various transformations this 

 plane character is mostly retained. In certain cases, how- 

 ever, the face of the leaf develops an outgrowth of tissues, 



Fio. 56. — A 

 phyllode of an 

 Acacia ; X §. 

 Often a few leaf- 

 lets of the com- 

 pound leaves ap- 

 pear at the tip. 



