334 PLANT PHYSIOLOGY 



has a very pronounced chemical or " actinic" action, of which the 

 germicidal effect of light is a familiar example. Strong light also 

 inhibits the action of certain enzymes, notably diastase, and this 

 may account for the fact that translocation and growth occur 

 largely at night even though the food is manufactured in the day- 

 time. Although the light of short wave lengths, i. e., the blue- 

 violet, checks growth, normal growth does not occur in the absence 

 of these rays from the spectrum, as shown by Shirley (1929). 



Polarized light apparently favors growth processes as shown by 

 Macht (1926-1929) and others. Macht found that under polarized 

 light the seeds of Lupinus albus germinated more rapidly and the 

 seedlings grew with improved vigor and at a higher rate than in 

 normal light. Some workers have attributed this favorable effect 

 to an action on photosynthesis. Macht finds that also the processes 

 of hydrolysis are favored, so that polarized light seems to improve 

 the general metabolism. Since a large part of the rays of moon- 

 light are polarized, this has been offered as an explanation of the 

 improved growth observed in many plants on moonlight nights, 

 but it is doubtful whether this explains the improved tonus of 

 college students under this environment. 



To bring about development and reproduction each species of 

 plant requires a certain number of light units. To a limited extent, 

 it is not important whether a dim light is supplied over a long 

 period of time or a bright light over a short period; the end result is 

 the same if there is an abundance of carbon dioxide present and if 

 light is the limiting factor. Bailey (1893) found that periods of 

 darkness are not necessary, and, by means of artificial light, 

 hastened the growth of lettuce by two weeks. Incandescent lights 

 are better than mercury vapor lights because of the greater amount 

 of red light which they contain. More recently Garner and Allard 

 (1920) by extensive studies have found that each species requires 

 these units to be given in a particular fashion, with the proper 

 alternation of light and darkness. If this natural rhythm of 

 illumination is changed, the tendency is to change the time of 

 flowering and reproduction, so that it is possible to control the 

 reproduction of the plant as well as the vegetative growth by regu- 

 lating the light (Chap. XXXI). Furthermore they have shown 

 that the effect of the length of day upon growth is not simply a 

 matter of total energy. Two short periods do not produce the 

 same effect as one long one. Shortening the light period much 



