120 RELATIONS OF PLANTS TO LIGHT 



which appears to have been developed for the purpose of acting 

 as a light-absorbing screen. The absorption by chlorophyl and 

 its derivatives, or accompanying substances, does not affect the 

 whole spectrum, and is greatest in seven different regions. The 

 energy derived from the radiations absorbed is used in splitting 

 apart the simple compounds taken up by the plant, and allowing 

 their unsatisfied chemical affinities to form new and more compli- 

 cated compounds of great potential energy, constituting the proc- 

 ess of photosynthesis. The construction and arrangement of the 

 organs of the plant to obtain the proper exposure of chlorophyl 

 has been the most important factor in the development of the 

 shoot. 



187. Critical Points in the Photosynthetic Relations of Light 

 to Plants. A minimum intensity of light below which energy 

 ceases to be absorbed and used is not easily distinguishable. The 

 diffuse rays of moonlight which have only the intensity of one six- 

 hundred-thousandth of daylight are doubtless sufficient to furnish 

 enough energy for some photosynthetic action, but it may not be 

 estimated since the amount of carbon dioxide used and oxygen 

 given off would be far overbalanced by the respiratory inter- 

 change. There is doubtless a minimum more or less adjustable 

 below which every species may not continue existence indefi- 

 nitely, but it does not lend itself to physical measurements. 



The optimum intensity for photosynthetic action is about that 

 of direct sunlight in the temperate zones. Generally a marked 

 increase over the optimal intensity must be made to exert a les- 

 sening effect upon photosynthesis. Reinke found that the in- 

 tensity must be increased sixty times before a decrease was 

 shown by Philotria. The maximum is equally intermediate with 

 the minimum, although it T is well known that any given species 

 cannot survive uninjured for any extended period in an intensity 

 above its accustomed standard. All of these critical points are 

 greatly influenced by other trophic conditions such as moisture 

 and temperature. The amount of light actually impinging upon 



1 Pfeffer, W. Plant Physiology, I : 340. 1900. 



