THE STUDY OF LEAVES. 



223 



openings may be found in the upper epidermis, but 

 ordinarily in much smaller numbers. 



The difference in the arrangement of the cells of the 

 upper and lower regions of the leaf is the cause of the 

 difference in the colour. It is owing to the presence of 

 air spaces below that the leaf is lighter here. In the 

 clover leaf light patches occur in the upper side which are 

 due to the presence of air. On placing the clover leaf in 

 boiling water the 

 light colour disap- 

 pears by the expul- 

 sion of the air. 



A s the green 

 colouring matter of 

 the leaf is thus 

 massed for the most 

 part towards the 

 upper side, we may 

 describe the upper 

 as the assimilating 

 surface and the 

 lower as the tran- 

 spiring surface. 



It is important 

 too to notice the 

 structural relations 

 between buds and leaves. Buds arise in the axils of 

 leaves. Pupils should examine a number of plants to 

 verify this point. They will then appreciate leaf modifi- 

 cation, e.g. spines, and will distinguish, by reference to 

 relative positions on the plants, spines which are modified 

 leaves and spines which are modified branches (Fig. 90) . 



Other modifications, such as tendrils, should also be 

 pointed out (Figs. 91, 92). Note may also be made of 

 the presence or absence of stipules blade-like expansions 

 at the base of the leaf stalk, and their degree of develop- 

 ment. Note e.g. the small adnate stfpules in rose, the 

 large leafy ones in pea tribe, including those of the Yellow 

 Vetch which take the place of the aborted leaf blade 

 (Figs. 92, 93). 



LEAF-TENDRIL 



Fig. 93. Leaves of Yellow Vetch (Lathyrus aphaca) 

 modified as tendrils. Stipules function here as 

 leaves. 



