THE DETERMINING FACTORS IN PHOTOSYNTHESIS 35 



ever, that heliophilous plants have an optimum light intensity 

 which is somewhat higher than that for shade plants, even though 

 Johansson has reported that the fern, Dryopteris austriaca, gave 

 maximum photosynthesis in 30% full sunlight and D. spinulosa 

 increased its photosynthetic activity up to 60% full sunlight,— 

 amounts rather high for heliophobes. 



Reinke (1884) provided us with a good illustration of the vari- 

 ation of growth with sunlight. He measured the production of 

 young plants from an older stem of Elodea canadensis and found 

 that the number of plants produced varied with the intensity 

 of the sunlight as follows: l 



Here we see that diminishing the light to one-fourth normal re- 

 duces the growth only one-half, while doubling the light has prac- 

 tically no effect on the number of plants produced. High in- 

 tensities of light diminish production through their destruction 

 of the chlorophyll (see below). 



In agreement with these general results, Boysen-Jensen found 

 that in forest trees photosynthesis increased with increasing 

 light until }io full daylight was reached; increasing the light 

 beyond this had no further effect. Lundegardh found that an 

 increase beyond 34 o fu ^ sunlight was without effect; but Stal- 

 felt has insisted that light is frequently a limiting factor in the 

 growth of trees. He found that pine and spruce absorbed more 

 carbon dioxide as the light was increased up until full daylight. 

 Similarly in fruit trees, he reports that in the sunlight they can 

 use about 50% more carbon dioxide than when in the shade. It 

 is possible that the reason for these differences lies in the fact 

 that in Sweden "full daylight" is not as intense as in regions 

 farther south. 



Plants need for growth and maturation a definite number of 

 light energy units, the unit being a product of the time and the 

 intensity. Within certain limits, a bright light acting for a short 

 length of time may have the same effect as a weak light acting for 

 a longer time, if there is abundant carbon dioxide present; but 

 as Davis and Hoagland (1929) have pointed out, as a result of 

 their experiments with wheat, the plant probably functions more 



1 Fraction of normal sunlight. 



