SIMPLE AND COMPOUND LEAVES 



91 



Monocotyledonous plants seldom have leaves with notched or 



cut margins, while dicotyledonous plants frequently have them. 

 A certain plan of venation is found 



mainly in plants with a particular mode 



of germination, of stem structure, and of 



arrangement of floral parts, and this is 



but one 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 should have netted-veined 



leaves, but many such facts as this are 



familiar to every botanist. 



110. Simple and compound leaves. 



The leaves so far studied are simple 



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 toward the bases of the veins. 

 In some leaves the gaps between the adja- 

 cent portions extend all the way down to 

 the petiole (in palmately veined leaves) or 

 to the midrib (in pinnately veined ones). 



FIG. 96. Pinnately g ucn divided leaves are shown in Figs. 95 

 divided leaf of -, n ~ 

 celandine and 96 ' 



Iii still other leaves, known as compound 



After Decaisne . , 



leaves, or branched leaves, the petiole, as 

 shown in Fig. 99 (palmately compound), or the midrib, as shown 

 in Fig. 97 (pinnately compound), bears -what look to be separate 

 leaves. These differ in their nature and mode of origin from the 



FIG. 95. Palmately divided 

 leaf of buttercup 



The blade of the leaf is dis- 

 continuous, consisting of 

 several portions. After 

 Decaisne 



