110 The Leaf 



this is that in a country as far north of the equator as 

 ours is they want as much light as they can get, and 

 they get more in that position than in any other. Tn 

 countries, however, like Australia, where the sky is 

 without a cloud for months together, the leaf blades 

 grow twisted on their stalks, so that only the thin edge 

 is towards the sky and the flat surface does not get the 

 sun's rays beating directly on it. 



You have probably noticed the perpetual trembling 

 of the leaves of the aspen tree and have been told 

 several pretty legends to account for this. The real 

 reason is only that the stalk is rather pinched from side 

 to side where it joins the leaf blade, with the result 

 that the blade is fastened on less stiffly than in other 

 trees and sways to and fro, even when there is no 

 perceptible wind to move it. 



In order to appreciate the part played by leaf stalks 

 in spreading out each blade to the light, you should 

 look at an ivy spray that has grown against a wall. 

 Here the leaves could not grow in their usual position, 

 with their flat sides facing the sky, because the wall got 

 in the way. To make up for this in some cases a stalk 

 has grown longer than the others, in others it has 

 twisted round so as to place the blade in a better 

 position. The result is that the lobes of each leaf fit in 

 between those of the ones next to it in a wonderful way 

 and form what is known as a leaf mosaic. This name 

 is given to the pattern because the effect is like the 

 mosaic pictures Italians make by fitting together small 

 pieces of stone. This same thing is seen in many elm 

 twigs, and some botanists think that the object of the 

 large and small side of this kind of leaf is to allow the 

 separate leaves to fit closely together (Fig. 55). A twig 



