PHOTOPERIODISM 689 



plants quickly flowered when exposed to a long day. Similarly Tincker 

 (72) maintained Trifolium pratense in a healthy, nonflowering condition 

 for 4 years by employing a light period of 10 hr. and Avena sativa exposed 

 to a short day was caused to function as a biennial. Kondo (38) obtained 

 vigorous growth but no development of panicles in rice plants exposed to 

 continuous light for a period of 3 years, although these plants developed 

 normally and formed seeds when subsequently exposed to favorable 

 day lengths. 



FORMATION OF TUBERS, BULBS, AND THICKENED ROOTS 



It was shown by Garner and AUard (23) that tuberization in its 

 various forms of expression is a prominent feature of photoperiodism. 

 Cultures of the McCormick, a late variety of potato, were exposed in one 

 case to the full daylight period of summer plus weak artificial illumination 

 from sunset till midnight, and in a second case to the full daylight alone. 

 The average temperature was relatively high. In a second test the 

 cultures were exposed to the full day and to day lengths of 5, 10, and 

 13 hr. With the artificially lengthened day no tubers were formed and 

 an underground bud, which otherwise might have given rise to a tuber, 

 failed to go into a resting state, but instead immediately germinated and 

 developed into a new plant. In each test the full day yielded a fair crop 

 of tubers, but the maximum absolute weight of tubers was obtained with 

 a 13-hr. day while by far the highest yield in percentage of total weight 

 of plant was produced by the 10-hr. day. The highest total weight of the 

 plants and the greatest plant height were obtained with the longest day. 



With a 10-hr. day Apios tuberosa produced relatively few but large 

 new tubers, while the mother tubers were much enlarged and formed 

 numerous prominent resting buds. With the full summer day many 

 new tubers of small size were developed and there was little enlargement 

 of the mother tubers which formed only a few small resting buds. With 

 Dioscorea divaricata L. a short day checked growth of the vine and 

 induced early tuberization of aerial axillary buds without materially 

 affecting relative production of underground tubers. In tests with the 

 yam, D. alata L., a 10-hr. day suppressed formation of aerial tubers but 

 greatly increased both the absolute and the relative yields of underground 

 tubers. With Helianihus tuberosus L. the short day produced small, 

 short-lived plants and while flower buds appeared, they were never 

 able to open. There was intensive tuberization, both the absolute and 

 the relative yields of tubers exceeding those produced by the controls, and 

 the numerous elongated underground stems produced by the latter were 

 lacking. The short day produced marked thickening of the root in 

 Phaseolus multiflorus Willd. 



The photoperiodic response of onion stands out in sharp contrast 

 with the preceding data on tuber formation. With a 10-hr. day the 



