XI] 



LEAF-MOSAIC 



119 



the position of the leaves and leaflets on their stalks are 

 such as to ensure a maximum of lighting and aeration for 

 each, and many of the peculiarities of petioles seem to be 

 intelligible on the assumption that swinging movements 

 from side to side e.g. the pendulum movements of Birch 

 leaves, and the lateral tremblings of Poplars enable the 

 otherwise partially shaded leaves to swing often into the 

 light, and to catch the breezes. 



But in many cases the shape of the lamina seems to 

 be fitted for similar advantages. For instance in the Ivy 

 (Fig. 27), Elm (Fig. 28), Lime, Hazel, &c, the base of 



Fig. '11. Prostrate shoots of Hedera Helix, Ivy, showing leaf-mosaic (K). 



the leaf is asymmetrical : whether the larger lobe in these 

 cases is a shelter for the young bud, or an adaptation to 

 fit the leaf-surface into the largest illuminated area is not 

 clear. It is conceivable that both events occur. 



Over-shaded leaves are often larger, and their palisade 

 layers are of less vertical depth, than normally illuminated 

 leaves : it may be that they thus obtain some compensation 

 for the feebler light they receive, by exposing a larger 

 surface to catch it, and a larger relative volume of spongy 

 mesophyll to make the most of the depressed assimilatorj 

 activity. 



