12 FUNCTIONS OF LEAVES [CH. 



others, including the Ivy (Fig. 27), when creeping on the 

 surface of the soil. 



There are other factors also of interest in these leaf- 

 mosaics, but they do not concern phyllotaxy, and will be 

 considered hereafter. 



Fig. 3. Portion of shoot of Abies, viewed from above and showing the 

 spirally inserted leaves combed to right and left in pseudo-distichous 

 series (D). 



It should, therefore, be observed that the adaptation of 

 these leaf-positions to expose as complete a leaf-surface as 

 possible to light and air, is the key to the whole subject. 

 Beautiful examples of this fitting in of leaf by leaf are 

 afforded by the Ivy and the Elm shown in Figs. 27 

 and 28. 



The typical leaf is, in point of fact, " a trap to catch 

 a sunbeam," and, as we shall see, its flat surface, thin 

 texture, manner of support on the shoot, and the mode of 

 exposure of the surface to the incident rays of the light 

 from the sun, are only a few of the adaptations to this 

 end, which are supplemented by many others all tending 

 in the same direction. True, there are other subsidiary 

 functions performed by leaves, which entail variations in 

 their structure and form under special life-conditions, but 

 these are not really opposed to the generalisation made 

 above : they are in fact supplementary to them, as we 

 shall see. 



