LIGHT AND VEGETATION 91 



the form of lenses which are said to concentrate the light 

 on the chloroplasts containing chlorophyll. 



Other plants are adapted to very high levels of illumination. 

 In fact, plants have been classified, according to observations, 

 as sun plants and shade plants. Cereals are examples of the 

 first category, woodland plants of the second. 



Each species is therefore adapted to an optimum illum- 

 ination. But rapid adaptations to changes of Ught intensity 

 have also been observed in the same plant in the course of 

 the same day. 



Besides the eucalyptus, which turns its leaves parallel to 

 the rays of the burning sun so that they make no shade, the 

 common bean is a striking example of this adaptation. In 

 the morning, the leaves are spread to capture as much 

 luminous energy as possible, but at midday, if the weather is 

 fine, they all turn on their petioles to present themselves 

 edgewise to the sun's rays, hke the eucalyptus leaves. The 

 illumination of the leaf surfaces thus remains moderate. 



Moreover, inside the leaf, the chloroplasts — green centres, 

 charged with chlorophyll, in which photosynthesis is effected 

 — change their position. In moderate Ught they spread out 

 parallel to the surface of the leaf, but in strong Ught they 

 arrange themselves at right angles to the surface so that those 

 nearest to it provide shade for the others. 



This internal modification can be observed, not only by 

 examination under a microscope, but also by the measure- 

 ment of the transmission factor; the quantity of Ught trans- 

 mitted increases in strong illumination because the absorbing 

 green particles are arranged in such a way that they allow 

 the luminous flux to pass more freely. This fact has already 

 been mentioned in the chapter on the photometry of the leaf. 



These reactions of certain plants to strong variations of 

 illumination are superposed on the variations of the per- 

 formance of photosynthesis itself; they have the eff"ect of 

 reducing, without suppressing, the variations of the quantity 

 of Ught available to the chloroplasts.' 



How does photosynthesis depend on this quantity of 



