FHOTOPERIODISM 699 



The interrelationship of intensity and duration of ilhimination also is 

 of interest, more especially in that the product of the two constitutes the 

 amount of light received by the plant. It will be recalled that Klebs 

 regarded the quantity of the light energy as of decisive importance. 

 It is now known, however, that this will hold true only for certain condi- 

 tions and usually within rather narrow limits. It has been made clear 

 that prolonging the daily light period by means of exceedingly weak 

 illumination may change the rate and character of growth quite out of 

 proportion to the quantity of light energy added. Shading tests (23) 

 with soy beans have shown that reducing the intensity of sunlight to 

 less than a third of the normal does not materially affect the time of 

 flowering. It is to be recalled, also, that midday darkening for several 

 hours and thus excluding more than 50 per cent of the total light energy 

 usually produces no important formative effects, whereas cutting off 

 less than 5 per cent of the light energy by early-morning or late-afternoon 

 darkening may cause definite formative action. 



A comparative study of the effects of intensity and duration of sun- 

 light on vegetative development of Raphanus sativus L. was made by 

 Johansson (32) in w^hich parallel cultures in the greenhouse were sub- 

 jected to day lengths of 6, 8, 10, 12 hr., and the full day, wath intensities 

 of 23, 39, and 70 per cent of full sunlight. Total growth increased with 

 increase in both the duration and the intensity of illumination though 

 not in the same degree from the standpoint of energy relations. The 

 root and the aerial organs responded differently to the two factors. The 

 former is more sensitive than the latter to increase in the light period. 

 With the highest intensity of light, maximum root weight was obtained 

 in the full day but with lower intensities in the 12-hr. day. Root growth 

 increased with increase in intensity of light with all day lengths, but 

 for the leaf this relation held only with the 6- and 8-hr. Ught periods. 

 The ratio of root development to total weight increased in all cases with 

 increased light intensity but was not further increased by lengthening 

 the day beyond 10 or 12 hr. 



Smith (69) concludes from an analysis of his results that, while both 

 the daily duration and the intensity of light influence the rate of develop- 

 ment as well as the amount of growth, the light period has the greatest 

 effect on rate of development, and light intensity chiefly affects the total 

 production of dry matter. This conclusion, within limits, appears to be 

 well supported by the data of other workers which already have been 

 discussed. 



Save for the observations of Oden, to which brief reference was made 

 in the discussion of supplementary artificial illumination, it appears 

 that as yet no detailed study of the interrelationship of spectral com- 

 position of light and length of day has been made. Comparatively little 

 has been done, also, as to interrelationship of humidity or water supply 



