THE LEAF 73 
Leaves are simple when there is only one blade, and compound 
when there is more than one. When the leaflets of a compound 
leaf are arranged on the side of a common axis (rachis), the leaf 
is pinnate (Fig. 55) and the leaflets are called pinne. If the main 
divisions are again once or twice pinnate, the leaf is bipinnate 
(Fig. 56) or tripinnate. 
One advantage of compound 
leaves is that they do not offer 
as much resistance to the winds 
as do entire leaves, and so need 
less strengthening tissue to 
protect them from being torn. 
The huge entire blades of the 
banana (Fig. 57) show an in- 
teresting modification in that 
they are not greatly strength- 
ened but instead have certain 
weak structural lines where 
the leaf can be torn by the wind 
without any particular damage Y 
to the plant. Fra. 58. Lobed leaf of papaya (Carica 
Plants having unbranched papaya). (x #) 
stems must have large leaves 
if they are to expose an extensive surface to the light. In 
keeping with this fact we find that large palms and tree ferns 
have enormous leaves. On the other hand, plants with many 
small branches have small leaves. 
The general shape of leaves is related to their arrangement 
on the stem. When leaves are densely crowded they are usually 
linear (long and narrow), and so shade each other much less 
than would broad leaves. When less crowded the leaves are 
more often somewhat oval in outline, and when spread out they 
are apt to approach a circular shape. 
When leaves are lobed, the lobing follows the large veins 
(Fig. 58), so that the photosynthetic tissue is nearer the princi- 
pal veins than would be the case if the leaves were entire. 

