EFFECTS OF LIGHT INTENSITY 735 



iioiisly in the dark, if grown from seeds or root stock well supplied 

 with reserve foods, develop excessively long petioles, and in mono- 

 cotyledons long leaf blades. The leaves of dicotyledons were shorter, 

 and all leaves were narrower in the dark. After placing in the light, all 

 leaves grew in breadth, and petiolate leaves grew also in length. Even 

 leaves which had ceased growth in the dark resumed growing when 

 placed in the light. The ability of the leaf or leaf part to renew growth 

 in light, after cessation in darkness, depended upon the relative age of the 

 tissue in question. The youngest tissues, relatively, grew the most, 

 and were able to resume growth after the longest period of interruption. 

 For this reason, leaves partially developed in darkness and then removed 

 to light never developed the same shape and relative proportion as leaves 

 exposed to daylight from the beginning. 



The effect upon etiolated plants of short daily exposures to light was 

 intensively investigated by Trumpf (113) and Priestley (76). Trumpf 

 found that the form developed by the bean plant Phaseolus multiflorus 

 depended upon the amount of radiation received, i.e., that within the 

 limits of his experiment, for a constant value of the product of intensity 

 times time a definite form of plant was produced. Plants exposed 

 for 3 hr. daily at 6600 meter-candles (613 foot-candles) had essentially 

 the same dimensions as those exposed for 40 min. daily at 30,000 meter- 

 candles (2787 foot-candles). However, this was not true of chlorophyll 

 content. The plants with long daily exposures to hght at low intensities 

 developed chlorophyll, whereas those which received the same amount of 

 light but in short exposures to high intensities developed no chlorophyll. 

 After 12 days' growth with 40 min. daily exposures to 30,000 meter- 

 candles, the plants were placed in daylight but still failed to produce 

 chlorophyll, while those exposed to low intensities produced abundant 

 chlorophyll in daylight. Excised leaves of the chlorotic plants developed 

 a deep green color and increased in size after 2 days' exposure to daylight 

 when placed in a weak solution of cane sugar. 



Trumpf further demonstrated that the influence of light in deter- 

 mining the form of the plant was independent of its photosynthetic action. 

 The evidence for this is that plants placed in an ice box at 7°C. and 

 exposed to light for short periods and also plants exposed, while under 

 the influence of narcotics, developed the same form as those exposed to 

 light in free air at room temperature, regardless of the fact that no 

 growth occurred while the plants were in the ice box or under the influence 

 of narcotics. 



Priestley (76) grew plants in light-tight chambers placed in an under- 

 ground cellar having no window. Special precautions were taken to 

 exclude extraneous hght. The chambers were provided with 0.5-watt 

 nitrogen-filled lamps of about 80 candle power, which were operated by 

 a timing switch so that four conditions of lighting were obtained, as 



