92 A TEXTBOOK OF GENERAL BOTANY 

and fold up while exposed to intense sunlight, such movements — 
seem to be fitted to reduce the rate of transpiration. The leaves 
of the sensitive plant (Mimosa pudica) not only show sleep 
movements but also fold up and hang down as a result of con- 
tact or of shaking (Fig. 90). When the plant is shaken, the 
leaflets of all the leaves may fold together and the pulvini of 
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Fig. 87. Marginal and stipular Fic. 88. Stipular spines of the 
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spines of the simple leaf of Acan- bipinnate leaf of cassia flower 
thus ilicifolius. (x 4) (Acacia farnesiana). (X 4) 
the leaves bend so that the leaves hang down. These move- 
ments result in a very striking change in the aspect of the plant. 
Instead of having a luxuriant display of foliage, it becomes 
very inconspicuous. The significance of these movements is 
not clear, but they would seem to afford some protection from 
browsing animals. 
Bud scales. We have seen that the tips of stems are pro- 
tected by leaves. We also find that many deciduous plants 
have specially modified leaf structures which protect the buds 
during the season in which the tree is leafless (Fig. 94). These 
structures are known as bud scales and may represent whole 
leaves, petioles with undeveloped blades, or stipules. Bud scales 
