688 BIOLOGICAL EFFECTS OF RADIATION 



Garner and Allard obtained maximum height in several short-day 

 plants with a long daylight period, but in Holcus halapensis L., another 

 short-day type, a 10-hr. day produced the tallest plants. In several 

 types of Dactylis glomerata, a long-day species, Tincker (71) obtained a 

 much greater height in the full summer day than with a 12-hr. day; and 

 with a 9-hr. day the plants remained in the leaf-rosette stage. Various 

 other long-day species showed more or less similar results. Using daily 

 light periods of 4, 6, 8, 10, and 14 or 16 hr. Lubimenko and Szeglova (42) 

 obtained maximum production of dry matter in Benincasa cerifera with 

 an 8-hr. day and in Phaseolus vulgaris with a 10-hr. day. In Momordica 

 Charantia, Gossypium herbaceum, Soja hispida, Hordeum vulgare, and 

 Sinapis nigra the 14- or 16-hr. period was more effective, and with a 

 10-hr. day the dry weight of Papaver nudicaule was only 8 per cent of 

 that produced by the 16-hr. day. The relative production of dry matter 

 per hour of daily illumination was highest with the 16-hr. light period in 

 Soja and Papaver, with the 8-hr. light period in Momordica, Gossypium, 

 Phaseolus, Hordeum, and Sinapis, and with the 6-hr. period in Benincasa. 

 Adams (1) obtained maximum height and largest production of dry 

 matter in flax, wheat, sunflower, white mustard, and soy beans with the 

 full day of summer. 



As regards the above experimental data Smith (69) emphasizes the 

 fact that for a given plant the relative effect of different light periods on 

 the rate and amount of growth varies according to the stage of develop- 

 ment at which the observation is made. Whether growth be measured 

 as accumulation of dry matter, plant height, leaf size, or rate of increase 

 in dry matter per day, per hour of illumination, or per unit of light, there 

 is found a maximum displacement, the maximum values for young 

 individuals of such species as oats, barley, rye, sugar pea, and tomato 

 being obtained with continuous light but undergoing displacement 

 toward the shorter light periods with increasing age of the plants. Appar- 

 ently the factor, i.e., length of day, has no fixed optimum for growth 

 reactions. Observed differences in growth reactions to day length 

 between different species cannot be considered as conclusive unless 

 comparison has been made on a basis of equal periods of development. 



It has been shown that many species may be maintained in a healthy 

 state of vegetative activity over long periods of time by subjecting them 

 to light periods which are unfavorable for reproductive activity. With 

 day lengths above the critical, typical short-day plants may attain giant 

 proportions. Similarly long-day plants exposed to day lengths shorter 

 than the critical for flowering may remain vegetatively active for long 

 periods, though usually total growth is relatively restricted. Garner 

 and Allard (26) were able to maintain certain species of Sedum in the 

 vegetative stage without flowering for as long as 9 years by exposing 

 them to daily light periods of 12 hr. or less. At the end of the test these 



