THE REQUIREMENT OF LIGHT BY PLANTS 



123 



garden crops at such times as will bring the development of those 

 plant ])arts that arc to he utilized at the season when the length of 

 (lay is most favorable. It should also enable florists to bring plants 

 into bloom at any desired time during the year. (Figs. 57, 58 and 



59.) 



74. The Requirement of Light by Plants. —There is no place in the 

 world where there is not enough light for plant life. Even in the 

 depths of the ocean and in underground caves there are saprophytic 



Fig. 59.— Oenothera biennis. Plants at left were exposed to the full daylight 

 period of the spring months. The individual at right received only ten hours 

 of illumination beginning March 19. It is seen that the short-day conditions 

 caused a decided weakening in power to elongate the primary axis, resulting 

 in development of numerous basal shoots. In this case dominance of the apical 

 bud is lost, a characteristic response to a suboptimal light period for elongation 

 of the stem. Photographed June 21. (After Garner and Allard.) 



bacteria and molds that bring about the decay of any organic matter 

 that may be there. Some of these saprophytic plants can spend their 

 entire lives in total darkness and some of them are readily killed by 

 exposure to light. All green plants, however, require a certain 

 amount of light. Chlorophyll, except in a few cases, is formed only 



