340 GROWTH OF PLANTS 



(11) Some plants (redwood, loblolly pine, Geum, hog peanut, etc.) will 

 survive in light intensities of 40 foot-candles. The sunflower needs much 

 higher intensities for survival. Redwood and loblolly pine survived for six 

 months in intensities that gave scarcely any increase in dry weight. In 

 light intensities up to 20 per cent of full sunlight the increase in dry weight is 

 proportional to the light intensity. Above that the dry weight does not 

 increase as fast as the light intensity. Chlorophyll content increases with 

 fall in light intensity down to about the minimal survival intensity. 



(12) Transpiration plays an important role in preventing overheating 

 of the leaf under high insolation. 



(13) The great range of environmental conditions discussed above modi- 

 fied the amount and proportions of the five chloroplast pigments: chloro- 

 phyll a and h, xanthophyll, carotene, and a broAvn pigment. 



(14) Lyssenko's conclusions on the vernalization of winter grains were 

 confirmed in the main, but it was found that spring cereals do not respond 

 to vernalization. 



- (15) Rosettes of Digitalis purpurea flower only after several months of 

 exposure to low temperatures, such as 40° to 50° F (4° to 10° C). Large 

 plants of Crassula ruhicunda showed similar response. In many cases low 

 temperatures during the night were sufficient to induce flowering. The 

 term thermoperiodism was adopted to cover the flower inducing response to 

 low temperature exposures. It seems to approach the importance of photo- 

 periodism in flower induction. 



Literature Cited 



1. Arthur, J. M., "Some effects of radiant energy on plants," /. Optical Soc. Am., 



18 : 253-263 (1929); also in B. T. I. Prof. Pap. 1 : 86-96 (1929). 



2. , "Red pigment production in apples by means of artificial light sources," 



C. B. T. /., 4 : 1-18 (1932). 



3. , "Plant growth in continuous illumination," in Duggar, B. M., editor, "Biolog- 

 ical effects of radiation," 2 : 715-725, McGraw-Hill Book Co., New York, 1936. 



4. , "Radiation and anthocyanin pigments," in Duggar, B. M., editor, "Biolog- 

 ical effects of radiation," 2 : 1109-1118, McGraw-Hill Book Co., New York, 1936. 



5. , "Day length, temj^erature, lamps and floriculture," in Internal. Gartenbau 



Kongr. 12th, Berhn, 1938, 2 : 1229-1237 (1939). 



6. , J. D. Guthrie, and J. M. Newell, "Some effects of artificial chmates on the 



growth and chemical composition of plants," Am. J. Bot., 17:416-482 (1930); 

 also in C. B. T. I., 2 : 445-511 (1930). 



7. , and E. K. Harvill, "Plant growth under continuous illumination from sodium 



vapor lamps supplemented by mercury arc lamps," C. B. T. I., 8 : 433-443 

 (1937). 



8. , , "Heating and lighting greenhouses with intermittent hght," C. B. T. I., 



10:15-44 (1938). 



9. , , "Intermittent light and the flowering of gladiolus and carnation," 



C.B.T.I., 11:93-103 (1940). 



10. , , "Flowering in Digitalis purpiirea initiated by low temperature and 



light," C. B. T. I., 12 : 111-117 (1941). 



