[Chap. XIV THE RATE OF PHOTOSYNTHESIS 117 



wide range of critical light intensity. Many fail to grow in the shade of 

 buildings or of other plants — for instance, in certain forests or beneath 

 some trees and shrubs on lawns — because the light intensity in such 

 situations is below their critical light intensity. Many of the small trees 

 one sees in the undergrowth of forests are merely surviving with a min- 

 imum of growth each \'ear; most of them finally die while still small. 



Plants that endure the most shade may be those in which maximum 

 photosynthesis is attained at low light intensities. Many more measure- 

 ments are needed to test this idea, but a few facts may be cited by way 

 of illustration. The maximum rate of photosynthesis in certain deep-sea 

 algae may occur at depths of 50 feet or more where the light intensity 

 is very low. When these algae are brought nearer the surface where the 

 light intensity is higher, the rate of photosynthesis is much lower. 



Similar data were obtained by a study of one of the shield ferns that 

 crrows in the dense shade of forests. Photosynthesis increased in this 

 fern with increase of light up to an intensity equal to one-fifth that of 

 full sunlight, but when the intensitv exceeded two-fifths that of full sun- 

 light the rate of photosvnthesis decreased. Photosynthesis in another 

 extreme shade fern (Trichomones) increased with increase of light up to 

 one-eighth of full sunlight and then decreased to none at one-half full 

 sunlisht. In the wood sorrel (OxaUs acetoseUa) , a small herbaceous 

 plant that grows well in the shade of northern forests, maximum photo- 

 synthesis was attained when the light intensity was only one-tenth that 

 of full sunlight, and this rate was maintained at higher intensities. 



Such plants as corn, wheat, apple, and spinach do not survive in dense 

 shade. In them photos\'nthesis is reported to increase with increasing 

 light intensity, in some cases up to full sunlight. When the measurements 

 are made on simple leaves, however, maximum photosynthesis is usually 

 found at intensities considerabl)' below that of full sunlight. When the 

 whole plant is tested the results are different, because many of its leaves 

 are shaded by others and do not get the full effects of increased light. 

 The greatest increase in photosvnthesis with increase of light occurs 

 when the original light intensitv is verv low. Measurements of photosyn- 

 thesis in relation to light and other external factors are now being made 

 bv a rapidlv increasing number of investigators. Many of the problems 

 being attacked are too complicated to be mentioned here. 



The owners of commercial o;ieenhouses are most interested in the 

 light intensitv that results in the best yields of flowers, fruits, and vege- 

 tables in different species of plants. During the cloudy days of winter, 



