498 RADIATION BIOLOGY 



various action spectra (Fig. 10-2). Similarly the action spectrum for 

 inhibition of second internode lengthening of Hordeum vulgare (Borth- 

 wick et al., 1951 ) is the same as that for enhancement of leaf development 

 of Pisum sativum (Parker et al., 1949). Other physiological responses 

 are also to be considered in their bearing on the similarity between initial 

 reactions in plants of different photoperiodic types. Moskov (1937) 

 found that flowering in the long-dark-period Maryland Mammoth 

 tobacco could be induced on short dark periods by grafting to it leaves 

 from Nicotiana rustica, which flowers independently of day length. Simi- 

 lar results were obtained with Soja max var. Biloxi, upon which leaves of 

 the variety Agate were grafted (Heinze et al., 1942). This latter variety 

 is also sometimes considered to be indeterminate, but it too is dependent 

 on long dark periods in that such periods give a more pronounced flower- 

 ing response. Control of flowering in plants of one photoperiodic type 

 by leaves of the opposite type was obtained by Melchers and Lang (1941) 

 in the case of Maryland Mammoth tobacco and Hyoscyamus niger. 

 Experiments of this kind strictly recjuire only that the floral-inductive 

 materials of short- and long-dark-period plants be effective for both types, 

 but generalization that a single material is involved is warranted as a 

 working hypothesis. Interspecific effects are also found in the depend- 

 ence of flowering of the parasitic species Orobanche minor on that of one 

 of its hosts, red clover (Holdsworth and Nutman, 1947). 



Reaction of certain strawberry varieties to the length of the dark 

 period affords an example of two distinct responses. Formation of flower 

 buds occurs only when the plants are subjected to long dark periods. 

 If the dark periods are shortened or are interrupted near the middle, 

 flower-bud initiation is inhibited, but runner formation is promoted. 

 Sexual reproduction of strawberry is thus induced by long nights, and 

 asexual reproduction, by short nights. Flower formation in strawberry 

 occurs first at the terminal of the main stem, its meristem forming a 

 terminal inflorescence. Following this the terminal meristems of buds 

 in the axils of leaves also produce inflorescence primordia. If photo- 

 periodic conditions are not conducive to flower-bud formation, these axil- 

 lary buds may develop as branch rosettes having very short stems and 

 producing a succession of vegetative leaves, or they may produce runner 

 plants at some distance from the parent plant through the great elonga- 

 tion of the first two internodes. Flower and runner formation thus 

 represent the extreme range of response of axillary buds to different 

 photoperiodic treatments. 



Variation within a species from short- to long-dark-period response 

 might also be indicative of control by the same initial reaction. Such 

 variation has been observed in a grass, side-oats grama, Bouteloua curti- 

 pendula (Olmsted, 1945). Cases of less extreme variation from varieties 



