500 RADIATION BIOLOGY 



1941). Several conditions, described from experiences of the authors, 

 might be met in subsequent development when a florally induced plant is 

 returned to an adverse photoperiod: (1) floral buds might fail to develop, 

 as in spinach, or abort, as in soybean; (2) incompletely formed flowers 

 might result, as in some varieties of sugar beet and Chrysanthemum; or 

 (3) the plant might continue to flower, as in Xanthium saccharatum, or 

 resume vegetative growth, as in soybean. 



There is a rapid recovery from the flower-promoting effects of long 

 dark periods on Biloxi soybeans; thus two successive photoinductive 

 cycles induce formation of flower primordia, but ten or more may be 

 required if each is alternated with a cycle unfavorable for flowering. 

 The effects of partial induction are well shown by development of 

 Euphorbia pulcherrima w^hen grown under dark periods near the critical 

 length (Parker, Borthwick, and Rapplej^e, 1950). Flowers develop with 

 bracts that are mottled green and red instead of intensely red, as on 

 plants that are adequately induced. Perilla (Moskov, 1939a) is another 

 example of a plant whose flower development is markedly dependent on 

 the length of the dark period. 



Short-day species and varieties differ greatly in the extent of their 

 recovery from effects of long dark periods; a species of cocklebur, Xan- 

 thium pensylvanicum, is an extreme case. A single dark period of ade- 

 quate length is sufficient not only to induce flower-bud formation in this 

 plant but also to enable the flowers to complete their development 

 (Hamner and Bonner, 1938). Lona (1947) found, however, that some 

 species of cocklebur are less responsive. A tetraploid race of X. italicum, 

 for example, required a minimum of two successive photocycles for floral 

 induction if the night lengths are about 15 hr and of more than two if 

 they are about 9 hr. The diploid form of the same plant regularly 

 responded to a single long dark period. Lona (1946) also observed that 

 X. orientale, although a long-dark-period plant in most respects, could 

 not be prevented from flowering by application of continuous light. 

 Similar observations Avere made for soybean varieties such as Agate and 

 Batorawka (Borthwick and Parker, 1939). Such a range of behavior is 

 probably of frequent occurrence in other groups of related varieties of 

 plants. 



The second question of possible leaf modification has been studied by 

 grafting "induced" leaves or branches on vegetative stocks or scions and 

 subsequently maintaining the entire plant under dark periods adverse 

 for flowering. Moskov (1939b) and Cajlachjan and Yarkovaja (1937), 

 Avorking with Perilla, found that a vegetative scion was induced to flower 

 when grafted to an induced stock. Similar results were obtained with 

 X. pensylvanicum by Hamner and Bonner (1938) when approach grafts 

 Avere made between induced and noninduced plants. This was not the 

 case for Soja max var. Biloxi (Heinze et al., 1942). The persistence of 



