GROWTH m CONTINUOUS ILLUMINATION 723 



latitude 69° 39', to Kingsbay, Spitzbergen, latitude 79°. He also used 

 artificial light alone and as a supplement to daylight. After considering 

 his own work and the published data of others, Smith came to some 

 very interesting conclusions: day length had an effect on "development" 

 or progress toward the flowering stage and also on "growth" as meas- 

 ured by dry-weight production, height of plant, intemodal length, and 

 leaf size. The rate of development per light unit for cereals (barley, 

 oats, and rye) reached a maximum on an 18- or 24-hr. day, while peas 

 reached a maximum on a 6-hr. day. Light intensity he found affected 

 both dry-weight production and development, but especially the dry 

 weight, while day length affected both rate of development and dry- 

 weight production, but especially rate of development. "Growth" 

 (measured by dry-weight production, etc., as already defined) per day, 

 per hour of illumination or per light unit was found to have a "maximum 

 displacement," that is, the maximum growth for young plants was found 

 on a 24-hr. day or continuous illumination, while the maximum growth 

 as the age of the plant increased was displaced toward shorter day 

 lengths. Smith found the "maximum displacement" generalization to 

 hold whether artificial or sunlight was considered. There is, therefore, 

 some evidence that long exposure to continuous illumination even in 

 sunlight operates in a w^ay which eventually checks growth and dry- 

 weight increase. This might indicate either that plants need a period of 

 rest in darkness of 5 to 6 hr. in each 24-hr. period, or that growth is 

 attuned to a progressive falling off in day length as the growing season 

 progresses. The latter relation would not appear extraordinary since 

 Gamer and Allard (10, 11) have already shown that plants are attuned 

 to flower on definite day lengths as the growing season progresses. 

 Using continuous artificial light, Arthur, Guthrie, and Newell (3) found 

 that the tomato developed the first signs of foliar injury in an exposure 

 of 5 to 7 days. This would indicate that the tomato needed a rest period 

 in each 24-hr. day rather than a progressive falling off in day length, 

 but since the quality of light used in this work is known to be more injur- 

 ious than sunlight, the evidence is not clear. It has been pointed out 

 in previous considerations that the tomato can be grown in continuous 

 sunlight within the Arctic circle. It may be, therefore, that even the 

 tomato will fit into the generalization that plants are attuned to a 

 progressive falling off in day length during the growth period starting 

 with continuous illumination when a light source is used which is less 

 injurious. 



GENERAL CONCLUSIONS 



It is evident from the foregoing discussion that growth rate and dry- 

 weight production of plants are rapidly accelerated by increasing the 

 length of day. This is true when either sunlight, artificial light, or 



