Ch. III. 9] SPECIAL Ft^'CTIOXS OF LEAVES 



81 



rather a kind of morphological entity easily specialized in 

 diverse directions. Recent investigations have sho■«^l 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 maj' 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- 

 like seeming stipules are 

 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 jerkj' 

 motion suggestive of the arms of the old 

 semaphore telegraph, — whence of course the 

 lets of the com- plant's name, 

 pound leaves ap- Tvpicallv. leaves are 



pear at the tip. . i . 



and in heir various transformations this 

 plane character is mostty retained. In certain eases, how- 

 ever, the face of the leaf develops an outgro^^th of tissues, 



G 



Fig. 55. — Leaf spines 

 of Barberrj- ; X 5. 

 (After Gray.) 



rV 



Fig. 56 

 phyllode of an 

 Acacia ; X 5. 

 Often a few leaf 



lat plates of tissue. 



