THE ROLE OF LIGHT 



351 



Even under a tree with a rather open crown the light intensity is only 

 one-tenth to one-twentieth that of full sunlight. Hence light is usually the 

 limiting factor for photosynthesis in most species of plants when growing in 

 the shade of trees. Species which normally grow in deep shade are probably 

 exceptions to this statement. On cloudy days light is also usually the limiting 

 factor in photosynthesis. Because of the prevalence of cloudy weather in many 

 regions during the winter, espe- 

 cially in December and January, 

 plants under glass often photosyn- 

 thesize at very low rates during 

 those months. The short length 

 of the daylight period also con- 

 tributes to low daily rates of 

 photosynthesis during the winter 

 season. 



A number of studies have 

 been made of the minimum light 

 intensities at which various species 

 of plants are just barely able to 

 survive. Unless reserve foods 

 have previously accumulated, the 

 minimum light intensity at which 

 a plant will remain alive for in- 

 definite periods during its nor- 

 mally active seasons must permit 

 sufficient photosynthesis to com- 

 pensate for both day and night 

 respiration, and in all probability 

 must also allow for some as- 

 similation. 



Bates and Roeser (1928) studied the effects of low light intensities upon 

 the growth of a number of species of evergreens native to the western United 

 States. The seedlings were exposed to a 200 watt, blue tungsten lamp for 

 10 hours a day. Differences in light intensity were obtained by growing the 

 seedlings at different distances from the source of illumination. Their results 

 (Fig. 89) show that redwood seedlings were able to maintain their initial 

 dry weight in a light intensity less than i per cent of full sunlight, while 

 pinon pine required about 5 per cent, and the other three species were inter- 

 mediate in their requirements. 



Light may exert indirect as well as direct effects upon photosynthesis. 



2 4 6 8 10 



Light intensity percent of full sunlight 

 Fig. 89. Relation between light intensity 

 and weights of conifer seedlings after nine 

 months. Data of Bates and Roeser (1928). 



