34 PLANT PHYSIOLOGY 



the minimum, the maximum, and the optimum. These terms are 

 self-explanatory. The cardinal points differ with the species, 

 some plants needing a great deal more light than others. Ferns, 

 firs, beeches, and many other plants of the deep woods thrive best 

 in not too bright sunlight and are called heliophobous (sun-fearing) 

 plants while others such as the pine, birch, and poplar thrive best 

 in bright sunlight and are called heliophilous (sun-loving). 



Not only is there a great variation in the intensity of light in 

 which plants thrive best, but an equally great variation exists in 

 the minimum intensity of light required. Thus Oxalis can grow 

 well in bright light, but it can also thrive in very weak light. 

 Sequoia sempervirens can use light as weak as 0.75% of sunlight, 

 but the Engelmann spruce and Douglas fir require a minimum 

 intensity of 1.2-1.5% and the pines of 2-6% full sunlight. The 

 fern, Adiantum capillus, was found in light 1/1700 as bright as 

 sunlight; and the algse of caverns can use light reduced to 1/2500 

 that of sunlight (Morton and Hofman, 1927). 



Schistostega osmundacea, an extreme heliophobous type, is a 

 moss which inhabits caves where it can grow in the very weak 

 light which penetrates to it. Like most shade plants, it is very 

 rich in chlorophyll, which may partly explain why it can thrive 

 so well in dim light. But Schistostega possesses other advantages: 

 the protonemal cells form a thin plate at right angles to the im- 

 pinging light and each cell is lens-shaped so that the light is con- 

 centrated on the chloroplasts, which lie in the bottom of the cell. 

 Thus what little light reaches the cells is well used. 



It has been shown experimentally that the algae of the ocean 

 can utilize the light of moonlight, and the fact that during periods 

 of full moon the ocean contains less carbon dioxide than during 

 the dark of the moon substantiates these results. In fact, at equal 

 intensities moonlight seems to be many times (100,000 times, 

 according to some!) as efficient as sunlight. This has been ex- 

 plained as due to the fact that about 10% of the moonlight is po- 

 larized. 



Light as a Limiting Factor. — According to Brown and Escombe 

 (1905) light is seldom a limiting factor in photosynthesis except, 

 of course, at night. For the average plant the light could be re- 

 duced to one-half of the intensity of bright sunlight without slow- 

 ing up the rate of photosynthesis, and many plants thrive well 

 which get only 2% of the available light. It seems probable, how- 



