50 The Living Plant 



used at the surface. Hence plants have had to distribute the green 

 tissues of the shoot in a manner ensuring the exposure of a great 

 spread of surface to light, and this involves a flattening of most 

 of the tissues of the shoot to the thinnest practicable structures. 

 This is why leaves exist, and why the green plant consists of them so 

 largely. 



Sixth. One of the essentials, the sunlight,, falls upon plants from 

 every direction in the aerial hemisphere. Not only does it come from 

 a source which forever is changing its position in the skies, but, 

 furthermore, this light is so strongly diffused through the atmos- 

 phere that it falls upon plants from every direction in an in- 

 tensity which for most of the time is as great as leaves can make 

 use of; for it is a physiological fact that plants cannot use all the 

 energy contained in full sunlight, and strong diffused light is 

 enough for their needs. Hence it comes to pass that plants 

 receive light in amount and direction sufficient to illuminate a 

 great many leaves if only these are carried to various heights and 

 spaced well apart, in a general distribution answering to that 

 of the incident light. This necessitates the specialization of a 

 part of the shoot for carrying the leaves upwards and outwards. 

 This is the reason why stems exist and branch in such manner as 

 typically to carry the leaves to a hemisphere of foliage. 



Thus it is evident that the most distinctive features of struc- 

 ture and form displayed by plants of the highest development, 

 the features indeed which are most closely associated with our 

 very idea of plants, the sedentary habit, the radial symmetry, 

 the diffuse-slender branching, the primary division into shoot 

 and root, and of the shoot into flat leaves and supporting stems, 

 all exist as adaptations which adjust the photosynthetic process 

 to the conditions under which the photosynthetic essentials are 

 supplied by the external world. It is therefore a fact that the 

 photosynthetic process determines the ground form and primary 

 structure of plants just as truly as it determines their ground 

 color. 



