LEAVES 



103 



of the frequent cases in botany in which the structures of 

 plants are correlated in a way which is not easy to explain. 

 No one knows why plants with two 

 cotyledons usually have netted-veined 

 leaves, but many such facts as this 

 are familiar to every botanist. 



121. Simple and Compound Leaves. 

 — The leaves so far studied are sim- 

 ple leaves, that is, leaves of which the 

 blades are more or less entii-ely united 

 into one piece. But 

 while in the elm 

 the margin is cut 

 in only a little way, 

 in some maples it 

 is deeply cut in to- 

 ward the bases of 

 the veins. In some 



leaves the gaps be- 

 tween the adjacent 

 portions extend all 

 the way down to 

 the petiole (in pal- 

 mately veined 

 leaves) or to the 

 midrib (in pin- 

 Such divided leaves 



Fig. 68. Pinnately Com- 

 pound Leaf of Locust, 

 with Spines for 

 Stipules. 



nately veined ones). 



are shown in Figs. 61 and 62. 



In still other leaves, known as com- 

 pound leaves, the petiole, as shown in 

 Fig. 67 (palmately compound), or the midrib, as shown in 

 Fig. 68 {pinnately compound), bears what look to be separate 



Fig. 69. Pinnately 

 Compound Leaf of 

 Pea. A tendril 

 takes the place of 

 a terminal leaflet. 



