LEAVES 1038 
of the frequent cases in botany in which the structures of 
plants are correlated in a way which is not easy to explain. 
Fic. 68. Pinnately Com- 
pound Leaf of Locust, 
with Spines for 
Stipules. 
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 sem- 
ple leaves, that is, leaves of which the 
blades are more or less entirely 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- 
Fic. 69. Pinnately 
nately veined ones). Such divided leaves — Compound Leaf of 
are shown in Figs. 61 and 62. Pea. A tendril 
In still other leaves, known as com- 
takes the place of 
a terminal leaflet. 
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: 
