c82 Journal of Agricultural Research voi. xviii, no. n 



revived, and serious efforts have been made to establish some form 

 of quantitative relationship between plant development and the quan- 

 tity of heat received from the sun. The work of Linsser (15, 16) and 

 of Hoffman (11, 12) in this field is worthy of special mention. In 

 this connection, also. Abbe's critical review of investigations having 

 to do with the relations between climates and crops is of interest (j). 

 It is believed that the results of the present investigation have an 

 important bearing on the subject. Since the quantity of solar radia- 

 tion received directly by the plant is the product of the intensity and 

 the length of the exposure, it might be expected that any relationship 

 existing between plant processes and the total quantity of radiation 

 received would be disturbed by changes in either the intensity of the 

 light or the duration of the exposure to its action. It has been shown 

 that the relative length of the day is a factor of the greatest impor- 

 tance in relation to reproductive processes in the plant, and it will be of 

 interest to consider whether the intensity of the solar radiation is also 

 of special significance. At the outset it may be observed that it hardly 

 seems likely that light intensity could exert a controlling influence on 

 reproduction in plants, in view of the extent to which the response of 

 plants to differences in light intensity has been studied by investigators 

 without discovery of any very significant relationships so far as con- 

 cerns reproduction. In the experiments discussed in preceding para- 

 graphs it was found that where daily exposures of 7 hours and 12 

 hours, respectively, were equally effective in shortening the vegetative 

 period, a total daily illumination aggregating on an average 9 to 10 

 hours but consisting of two separate exposures, with a 4-hour period 

 of darkness intervening, was vastly less effective in this respect. This 

 shows at once that the total quantity of radiation received can not 

 be responsible for the shortening of the vegetative period produced 

 by shortening the single daily exposure to light. Furthermore, since 

 in the double daily exposures the intervening period of darkness to 

 which the plants were subjected, 10 a. m. to 2 p. m., was at the time 

 of day when the intensity of the solar radiation reaching the earth's sur- 

 face is at its maximum, the average intensity of the radiation received 

 by these plants is less than that received by those plants which were 

 exposed continuously from 9 a. m. to 4 p. m. 



The Stewart Cuban Mammoth tobacco which requires a day length of 

 12 hours or less to attain the blossoming stage has been grown commer- 

 cially to some extent under an artificial shade of coarse cheesecloth 

 estimated to reduce the intensity of the sunlight by approximately 

 one-third. It has been observed that this shade has had no noticable 

 effect on the date of blossoming of the tobacco. Again, the aster used in 

 the present investigation grows in the wild state under a variety of situ- 

 ations, some of which are very shaded, but observation during the past 



