ENERGY: ILLUMINATION 115 



exiguous supply of carbon dioxide always limiting the amount 

 of assimilation. If the leaf is better supplied with raw 

 material, it makes a better show as an economic factory of 

 organic matter. In an atmosphere with 5 per cent, carbon 

 dioxide, Willstatter and StoU (19 18) measured an assimila- 

 tion ten times as great as that obtained by Browne and 

 Escombe. If this took place in sunlight, about 10 per cent, 

 of the incident energy would be employed ; even under such 

 conditions the waste is enormous. 



§ 10. Orientation of the Leaf and Illumination 



The leaf is, however, not always exposed to bright 

 sunlight ; in such climates as ours bright sunlight is rather 

 the exception. It works late and early, in mist and under 

 dark clouds ; owing to the presence of other vegetation it 

 may work habitually in deep shade. We find that it is 

 usually so oriented as to absorb efficiently what light may be 

 available, even though occasionally or frequently the supply 

 may be much in excess of what it can utilise. Its broad 

 expanded surfaces must, how^ever, be taken not only as 

 serving for the absorption of the energy, which is abundant, 

 but also as making for efficient utilisation of raw material, 

 the carbon dioxide, which is scarce. 



Even a cursory examination of the leaves of a few shrubs 

 and trees shows that the blades are so disposed as not only 

 to receive good illumination individually, but also to avoid 

 mutual shading. The primary arrangement of the leaves 

 with reference to the shoot axis is usually very regular ; 

 this phyllotaxy may be referred to a comparatively small 

 number of ground types. A species has a very constant 

 leaf arrangement, though this may change from one type to 

 another during development. The final position assumed 

 by the leaf blades is, however, determined largely by the 

 direction of incidence of light, and may completely mask the 

 nature of the original relation to the shoot. The position 

 of the mature blade is attained by bending and twisting 



