Mar. 1, 1920 Effect of Length of Day on Plant Growth 555 



work on the subject. In most instances stems, and frequently leaves, 

 exhibited negative geotropism in the absence of light. In all species 

 investigated etiolated tissues show a lesser degree of differentiation than 

 the normal. In this connection MacDougal points out that the dif- 

 ferences exhibited between etiolated specimens and normal plants 

 demonstrate the fact that growth, or increase in size, and develop- 

 ment, or differentiation, are distinct processes capable of separation. 

 For present purposes perhaps his most important observation is that in 

 no plant investigated had the stamens and pistils attained functional 

 maturity. 



The effects of differences in the length of the daylight period, the sub- 

 ject of the present study, have not been so extensively investigated as 

 most other phases of light action. Obviously the problem may be 

 approached in any one of four ways : by comparing the behavior of plants 

 when propagated in different latitudes, by growing plants at different 

 seasons of the year in the same latitude, by supplementing the daylight 

 period with artificial light, and by preventing light from reaching the 

 plant for a portion of the normal daylight period. In the records of 

 attempts to grow various plants in different parts of the world there are 

 undoubtedly a, great deal of available data bearing on the present prob- 

 lem; but apparently no systematic effort has been made to utilize this 

 material, the reason probably being that the importance of the relative 

 length of day in affecting plant processes, and, in particular, reproduc- 

 tion, has not been appreciated. Bailey (j, 4, 5,) carried out an extensive 

 series of tests in which daylight illumination was supplemented by the 

 electric arc light applied for different portions of the night. The addi- 

 tion of the artificial light induced blossoming and seed formation in spin- 

 ach. The additional light also favored the growth of lettuce. Rane 

 (20), using the incandescent filament electric light, and Corbett (7), 

 employing incandescent gas light, observed that certain flowering plants 

 and some vegetables blossomed somewhat earlier when the normal 

 daylight illumination was supplemented with artificial light. In most 

 of these tests the artificial light was applied for the entire night, but ap- 

 parently the results so far as concerns reproduction were essentially the 

 same as when the plants were darkened for a portion of the night. Tour- 

 nois {24, 25) has reported the results of an interesting experiment with 

 hemp {Cannabis sativa L.) and a species of hops (Humulus japonicus 

 Sieb. and Zucc.) in which these plants were exposed to sunlight only 

 from 8 a. m. to 2 p. m. daily. It had been shown by Girou de Buzarein- 

 gues (10) as early as 1831 that when planted in the late winter or very 

 early spring months the hemp plant first develops in the spring a number 

 of abnormal sterile blossoms in the leaf axils and later produces normal 

 flowers at the regular blossoming period. Following up this fact Tournois 

 concludes from the above-mentioned experiment that the abnormal 



