720 BIOLOGICAL EFFECTS OF RADIATION 



and the intensity of light produce characteristic changes in chlorophyll 

 pigmentation and internal structure of leaves and other plant organs, so 

 that the differences observed by Bonnier in the case of plants grown 

 under continuous illumination cannot be attributed wholly to the 

 continuity of the light. Since Bonnier did not record the intensity of 

 illumination in his experiments, it is possible that the highest intensity 

 used was yet sufficiently low to produce some etiolation effects, although 

 considering the distance of the lamps from the plant (0.5 meter) this does 

 not seem probable. Maximov (17) studied the growth of plants under 

 continuous illumination using 500- and 1000-watt filament lamps. He 

 compared the leaf structure with that of plants grown under alternating 

 illumination of 12 hr. light and 12 hr. darkness and found greater differ- 

 entiation of palisade and mesophyll tissue under continuous illumination. 

 This is in direct contrast to the work of Bonnier. 



Pfeiffer (19) studied the structure of leaves and stems of plants grown 

 both under continuous artificial light and on a 19-hr. day, 12 hr. of which 

 were sunlight. Incandescent filament lamps of 1000- or 1500-watt 

 current consumption were used in this work. She found that continuous 

 artificial illumination reduced the thickness of leaves ; while some varieties 

 of plants grown on the 19-hr.-day conditions had thicker leaves, others 

 had thinner leaves. Higher carbon dioxide concentrations tended to 

 increase leaf thickness. The thickness of the palisade layers of cells in 

 general followed closely the leaf thickness, showing less palisade develop- 

 ment in thinner leaves. No marked variations in the spongy mesophyll 

 layers of cells or in the epidermal layers were noted. Stomatal counts 

 were highest under the 19-hr.-day conditions with higher carbon dioxide 

 concentrations and least imder continuous artificial illumination. In 

 the case of buckwheat plants grown on 5, 7, 12, 17, 19, and 24-hr. days of 

 artificial light, the maximum height and stem diameter were reached 

 on a 17-hr. day; also the maximum amount of xylem development. 

 Both the 19-hr. day and continuously illuminated plants, as well as 

 those grown on shorter day lengths, showed correspondingly less of the 

 highly differentiated tissues. 



Arthur, Guthrie, and Newell (3) studied the growth of several species 

 of plants under artificial light alone and in combination with sunlight. 

 An attempt was made in this work to grow plants throughout their life 

 history with photosynthesis at or near its maximum rate by supplying 

 a high light intensity and long day along with increased carbon dioxide 

 concentration and a relatively high temperature. A chemical analysis 

 of many of the plants was made to determine the carbohydrate relations 

 to increasing length of day and increasing light intensity. Height 

 growth and weight of tissue produced were also recorded. In general, 

 carbohydrates and weight per plant increase with day length up to the 

 point where foliar injury begins to be effective in holding the plants 



