690 BIOLOGICAL EFFECTS OF RADIATION 



Silverskin onion remained in the vegetative stage without indication of 

 flowering or of forming bulbs over a period of more than 12 months. 

 With the full summer day the plants flowered and formed large bulbs 

 which soon passed into the resting stage but resumed vegetative activity 

 when the day length had shortened sufficiently in autumn. 



The relation of day length to tuber formation also has been studied by 

 several other investigators. McClelland (47), under Puerto Rican 

 conditions, obtained a sevenfold increase in production of tubers in the 

 Lookout Mountain variety of potato with a 10-hr. day as compared with 

 a 15-hr. day, and a smaller increase in the Irish Cobbler while the yield 

 was not affected by the light period in the Red Bliss. In all cases, how- 

 ever, the ratio of tubers to tops was vastly greater with the short day 

 than the long day. Growth of tops was much greater in the long day. 

 The light requirements for bulb formation in different varieties of onions 

 was found to be highly specific, but in all varieties a short day prevented 

 formation of bulbs and favored leaf growth. Zimmerman and Hitchcock 

 (78) observed that in six varieties of Dahlia the length of day determined 

 the type of root system formed, storage roots being correlated with a short 

 day and a fibrous root system with a long day. According to Doro- 

 shenko and associates (18) the normal summer day length in the region 

 of Leningrad completely suppressed tuber formation in several forms of 

 Ullucus and Ozalis and in South American species of Solanum. While 

 all native cultivated varieties of potato were capable of forming tubers 

 under the long summer day, both the yield and the number of tubers 

 formed were increased with a short day. The optimum light period 

 for tuber production was found to be 9 to 12 hr. The process of tuber 

 formation shows a specific dependence on length of day. Schick (64) 

 at latitude 52.5° north has reported similar results with potatoes. 



In contrast with the foregoing results Arthur and others (5) find 

 that in the Irish Cobbler variety of potato formation of tubers is favored 

 by long days when associated with low temperature and high intensity of 

 light. Undoubtedly low temperature often favors tuber development in 

 the potato, but with respect to the day-length factor it must be concluded 

 that the relation to tuber formation is largely a matter of the variety 

 and species. This seems reasonably clear from the previously mentioned 

 results of other investigators at various latitudes. 



It is apparent that the daily light period is an important factor in the 

 formation of tubers, bulbs, and thickened roots. In this type of response, 

 as in the case of sexual reproduction, plants vary in their sensitivity to 

 the day-length factor. Moreover, among the more sensitive plants 

 the optimum day length for this type of development varies with the 

 species and with the variety. Practically without exception, however, 

 under normal growing conditions investigators have obtained the maxi- 

 mum relative production of tubers (and thickened roots) with a short 



