THE STRUCTURE OF THE SHOOT 63 



of perceiving or appreciating differences of intensity o^ 

 illumination. This sensitiveness is of the greatest value 

 io the shoot, for as the stem bends towards the source of 

 the light the leaves which are expanded nearly at right 

 angles to it are exposed to the rays which they need for 

 the manufacture of sugar. 



The leaves also manifest an independent sensitiveness 

 to light. They are generally so expanded as to expose 

 their upper surfaces to the sunshine. If this position 

 cannot be attained without a movement of the leaf this 

 movement is effected and supplements the other. The 

 leaf-blade twists on its petiole, or the petiole twists in 

 such a way as to expose the surface of the blade. 



With the same sensitiveness to light we see thus that 

 the different members of the plant respond differently, 

 but always purposefully, to it. The root grows away 

 from the incident rays, penetrating into the deeper 

 crevices of the soil ; the stem grows towards them, 

 while the leaf places itself across their path. 



The positions assumed by the stem, branches, and 

 leaves are greatly influenced by the various stimulations 

 they receive ; some respond more actively to one, others 

 to another; but all show both perception and response 

 as they adapt themselves to their environment. 



CHAPTER VII 



THE STRUCTURE OF THE SHOOT 



WE must now examine what are the arrangements of 

 the internal structures of the shoot which enable it to 

 carry on these duties. Though the shoot is to be 

 regarded as a single system comparable with the root, 

 the duties discharged by its cylindrical and its flattened 

 parts are so far distinct that it will be well to consider 

 them separately from our present point of view. 



