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 entirely united 
Fic. 68. Pinnately Com- 
pound Leaf of Locust, 
with Spines for 
Stipules. 
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- 
nately veined ones). Such divided leaves 
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 
Fie. 69. Pinnately 
Compound Leaf of 
Pea. A tendril 
takes the place of 
a terminal leaflet. 
