752 BIOLOGICAL EFFECTS OF RADIATION 



THE INFLUENCE OF LIGHT INTENSITY ON FLOWERING 



Under extremely low light intensities, plants may develop leaves 

 but they do not flower. The minimum values required for flowering 

 have not been accurately established. Apparently at least 10 per cent 

 of normal summer sunlight of temperate regions is required for the flower- 

 ing of most plants. The plants which commonly grow in the shade are 

 able to flower at lower intensities than those normally accustomed to 

 full sunlight. Ordinarily, shading does not appreciably delay the time 

 of flowering, unless it is so dense as to seriously interfere with the nutrition 

 of the plant. Shading often tends to prolong the vegetative and fruiting 

 period, while full sunlight tends to hasten maturity. 



Gourley (41) found that flowering of apple trees shaded in the spring 

 before the buds opened was not appreciably interfered with by the shade, 

 but the next season, after a full year of shade, it was considerably reduced, 

 while the third season two trees produced only eight clusters. Reduc- 

 tion in flowering was likewise observed for tomato, geranium, and nastur- 

 tium. Similar reduction in flowers and delay in flowering were noted by 

 Auchter et at. (5, 6). 



Daniel (26) found that shading head lettuce not only prevented the 

 formation of the typical head but also caused a decrease in flower heads 

 from 3287 to 425 and in number of fertile achenes per head from 24 to 

 13. Similar reduction in number of flowers and fruit in shaded plants 

 have been observed by Zillich (122), Shantz (93), Shirley (94), and Yin- 

 son (117), as well as by many ecological workers. Schrader and 

 Marth (90) found that shading individual apple fruits caused an inhibition 

 of red-pigment development and also some reduction in size of fruit. 



THE INFLUENCE OF LIGHT INTENSITY UPON TRANSPIRATION, WINTER 

 HARDINESS, AND RESISTANCE TO DROUGHT 



Conducting a very comprehensive series of experiments on the 

 transpiration of crop plants, Briggs and Shantz (15) made daily records 

 of radiation, temperature, wet-bulb depression, and wind velocity. 

 These were correlated with the daily rate of transpiration over a period 

 of two years for a wide variety of grains and leguminous plants. The 

 correlation ratio between radiation and transpiration varied from 0.65 for 

 small grains to 0.48 for leguminous crops. Individual varieties showed 

 ratios as high as 0.80. They showed that of the radiation received, an 

 equivalent of 50 to 100 per cent was dissipated in transpiration. Like- 

 wise, Penfound (72) observed that Helianthus annuus plants developed in 

 full sunlight, evaporated 3610 cc, as contrasted with 821 cc. evaporated 

 by the same species in a shade providing 20 per cent of full sunlight. 

 For Polygonum hydropiper the values were 3125 and 1190 for sun and 



