SUN RAYS AND PLANT LIFE— JOHNSTON 



361 



allel with the sun's rays and receive a minimum amount of direct 

 radiation. 



Even the interior of leaves frequently undergoes structural changes 

 with increasing or decreasing light intensity. The microscope re- 

 veals in some instances a change in position of the chloroplasts within 

 the leaf cells, as illustrated in figure 3. These chlorophyll-containing 

 bodies arrange themselves across the path of weak beams of light as 

 shown in the upper figure, a. In strong light these bodies migrate 

 to the side walls, thus permitting a minimum amount of exposed 

 surface, h. 



PiGUKE 3. — Diagram showing positiou of plastids in cross-section of a leaf (a) lu diffused 

 light and (6) in intense light. Arrows indicate direction whence light is coming. 

 After Stahl. 



All increase of dry weight in plants depends on their assimilating 

 carbon dioxide from the air under the influence of light. All the 

 carbon in coal and wood, grains, oils, and many other indispensable 

 products comes, in the final analysis, from this source and depends 

 for its energy content on sunlight. In figure 4 the gas exchange 

 between a green leaf and its immediate environment is represented. 

 It will be noted that while respiration goes on continuously in light 

 and darkness, photosynthesis takes place only in light. 



Sunlight intensity varies under natural conditions from at night 

 to as much as 10,000 foot-candles on a bright summer day. Most 

 plants grow very well in intensities considerably under the high 

 figure just noted. In experiments with artificial light good growth 

 has been obtained with intensities as low as 2,000 to 3,000 foot-candles. 

 Numerous experiments carried out by William H. Hoover at the 



