THE LEAF OL 
feeding of animals on the foliage or stems can be regarded only 
as harmful to the plants. Many plants are protected from ani- 
mals, to a greater or less degree, by the development of spines 
(Fig. 87). On the other hand, 
many spiny plants are eaten 
by browsing animals. Spines 
may be stem or root struc- 
tures, but very frequently they 
are modified leaves. Whole 
leaves that are modified as 
spines (Fig. 44) may or may 
not serve for photosynthesis. 
The spines of the cacti are 
believed to be modified leaves 
(Fig. 512). In some cases 
spines are developed at the tip 
or on other portions of a leaf. (WV 
The daggerlike leaf of the if 
century plant is a striking ex- 
ample of a leaf whose tip is de- 
veloped into a spine. Stipules 
are sometimes modified into 
stout spines (Fig. 88). 
Motile leaves. The leaf bases 
_ of many leaves and the bases of Fie. 86. Stipules of India rubber tree 
the leaflets of many compound (Ficus elastica) 
leaves are modified into motile The united stipules of the young open- 
organs known as pulvini. These ing leaf surround the bud, while those 
: : of the second leaf are about ready to 
are most conspicuous In com- fall. (x 4) 
pound leaves, where the whole 
leaf hangs down and the leaflets fold together at night (Fig. 89) 
or when exposed to intense sunlight. Such movements are the 
result of the bending of the pulvini. The significance of the 
drooping and folding up at night (sleep movements) is not under- 
stood, but it has been suggested that these movements serve to 
reduce the radiation of heat from the leaves. When leaves droop 




is 



