418 Plant Physiology 



It has been reported that during a unit interval the even- 

 ing primrose utilizes in direct sunlight about three times 

 as much CO 2 as when in diffuse light, while the common 

 polypody works more effectively in the latter. Many 

 shade-loving plants may reach maturity in light which 

 is reduced to about -5^ the intensity of maximum sunlight. 

 Beyond a certain light intensity the plant gains little, for 

 the small amount of CO 2 in the air is then the limiting fac- 

 tor in growth. Temperature is a further limiting condi- 

 tion. In the warm forest of the tropics there may be a 

 vigorous forest-floor vegetation, but the cold shade of a 

 far northern forest affords only a scant undergrowth. 



254. Light intensity. Upon the surface of the earth 

 light intensity varies considerably both diurnally and 

 seasonally, depending, of course, with a clear sky, upon the 

 altitude of the sun. The possible daily maximum is at 

 sun-noon, June 22. If this intensity should be repre- 

 sented at the equator by 100, then with a growing season 

 in the north temperate zone approximately from March 

 21 to September 23, or its equivalent in the southern 

 hemisphere, the light intensity from 9 A.M. to 3 P.M. 

 would be represented approximately by 82 to 98 and the 

 noon variation by 93 to 98, as calculated by Clements for 

 Lincoln, Neb. From this it is apparent that the variation 

 in light intensity throughout agricultural regions with a 

 clear sky during the growing season is not considerable. 

 Range of intensity in the open is in general insufficient 

 seriously to affect vigorous growth, although it may mod- 

 ify form and chemical content. In some regions cloudi- 

 ness may be an important factor. 



255. Injurious effects. It has long been well estab- 



