188 Light 



to grow rather widely spaced, and the relatively sparse foliage allows 

 many patches of sunlight to reach the ground. The canopies of pines 

 and oaks usually have fewer gaps. Measurements made in Illinois 

 revealed that the portion of the forest floor exposed to direct sunlight 

 was 84 per cent for poplar, 77 per cent for pine, and 35 per cent for 

 oak. In elm-maple forests and in tropical rain forests the canopy 

 typically is complete, with the result that none of the direct sunlight 

 reaches the forest floor. Other measurements showed that the light 

 on the forest floor which had passed through the leaves of the canopy 

 was reduced in intensity to 3.5 per cent of its value above the tree 

 tops in the oak forest; corresponding figures for elm-maple and rain 

 forests were 0.4 per cent and 0.2 per cent, respectively. Thus less 

 than 1 per cent of the light outside the forest reaches the ground if 

 the canopy is complete. 



Different types of forests exhibit different seasonal influences on the 

 light factor. Underneath a stand of pine trees the light is reduced 

 by about the same amount throughout the year since the trees are 

 evergreen. The illumination on the floor of a pine forest reaches a 

 maximum in the early summer and a minimum in winter correspond- 

 ing to the seasonal variation in the intensity of the incident sunlight. 

 In a maple forest, however, a very different seasonal picture is pre- 

 sented. During January, February, and March the plants and ani- 

 mals living on the forest floor receive increasing amounts of light, but 

 in April the leaves begin to appear and the intensity of light drops 

 rapidly reaching a minimum in the middle of the summer. Then, as 

 the leaves of the maple trees begin to curl and drop off a larger 

 amount of light is allowed to filter through the forest canopy. The 

 striking seasonal variation in the amount of ultraviolet radiation pene- 

 trating different forest types is shown in Fig. 6.2. The changes in the 

 total visible illumination are of a corresponding nature. ( Park, 1931 ) . 

 Accordingly, of you were a beetle, or, if you prefer, a lily growing in 

 a maple forest, the brightest month of the year for you would be April 

 and the darkest would be July- In this situation living organisms have 

 modified the environment in respect to the light factor so that the 

 seasons have been practically reversed. 



Duration and Amount of Light. The total amount of light received 

 by an organism is determined both by the intensity of the light and 

 the duration of the period of irradiation. The situation is similar to 

 the exposure of a photographic plate. The amount of blackening of 

 the plate is determined by the intensity of the light times the period 

 of exposure. In natural habitats, the variation in the length of day 



