138 PLANT LIFE. 



Natural History is totally different from the arrange- 

 ment described here, except as regards the four end 

 leaves of the branch. At the junction of two side 

 twigs, the leaves are displaced, and have entirely altered 

 their size, so that small ones are arranged between the 

 larger. The result is a very pretty piece of mosaic 

 work, and illustrates very well the important fact that, 

 though Nature works on hard and fast mathematical 

 lines, room is always left for special necessities, and 

 free play is allowed for variations adapted to the un- 

 foreseen. The development of the leaves depends, as a 

 matter of fact, on the amount of light which they 

 obtain and this limits their growth. Each leaf has also 

 its own motion, and is not held to one special position, 

 or hard and fast form. 



At first, it is not easy to understand a very simple 

 cause of confusion. The light is very seldom decidedly 

 overhead. It is often entirely from one side ; or it may 

 be diffused and broken up by other leaves, and this 

 alters at once the arrangement, and even the shape of 

 the foliage. A sort of leaf very common in climbing 

 plants, and of a very neat mechanical structure, is 

 difficult to bring under any law whatever. The leaf of 

 a Convolvulus or any ovate, acute or aaiminate leaf, heart- 

 shaped at the base, will explain this type. Yet it is 

 not difficult on examination to find the theory of this 

 shape. The tip fits either into the space between two 

 neighbouring leaves, or into the little notch at the base 

 of one below, and thus saves light. 



The light-catching effect of the whole leaf again 

 depends, not on its actual shape, but on the amount of 

 light which it intercepts ; or, we might say, on the 

 shadow which it throws. If a dandelion leaf is copied 

 on cardboard and the latter cut out to the shape, then 



