Vernalization and Photoperiodism — 44 — A Symposium 



and despite the shortened season. But southern plants are more difficult 

 to adapt to northern latitudes where, due to the much longer days, they will 

 continue to grow vegetatively till killed by frost. 



Within specific groups of plants there may exist great variability as to 

 their responses to definite photoperiods, even if attention is paid only 

 to flowering and not to the earlier or later stages of the reproductive cycle. 

 Thus within the genus Phaseolus, to three species of which (P. vulgaris, 

 P. lunatus and P. coccineus) belong the beans commonly grown in the 

 United States, there is a large number of varieties that are either short- 

 day or day-neutral. But the Scarlet Runner bean, a variety of P. coccineus, 

 is a long-day plant. Since most of the varieties of P. vulgaris and P. luna- 

 tus are day-neutral, they can be and are grown over a considerable range 

 of latitudes. In fact, much of the varietal improvement in these two spe- 

 cies has been associated with day-neutral characteristics (Allard and 

 Zaumeyer, 1944). Analogous and possibly even more extreme situations 

 are exhibited by soybeans (Borthwick and Parker, 1939; Rudorf and 

 ScHROCK, 1941) and many other cultivated species (Beljdenkova, 1940; 

 Goodwin, 1941 ; Laibach, 1940; Allard, 1941). 



Among wild plants, or plants that have been domesticated but little, simi- 

 lar conditions seem to exist. Of the several instances that could be cited, 

 it would seem to be sufficient to refer here to two recent studies. Olm- 

 sted (1944-1945) has investigated, both in the field and laboratory, the 

 photoperiodic response of twelve geographic strains of side-oats grama 

 grass {Bouteloua curtipendula) . They were found to differ in their re- 

 sponses to the length of day, showing the existence of definite photoperiodic 

 types within the species. Strains from southern Texas and Arizona con- 

 sisted largely of short-day or intermediate plants, those from Oklahoma, 

 Kansas and Nebraska included numerous long-day individuals, and those 

 from North Dakota were made up chiefly of long-day plants. It is con- 

 cluded that most specimens, within the twelve strains investigated, probably 

 can develop flowers best on photoperiods existing in their native habitats. 

 Whether this wide variability in response to the photoperiod is due to more 

 or less stable inherited characters or is the result of heterozygocity, is not 

 certain from the evidence presented. A similar situation seems to exist 

 in other groups of plants (Rasumov, 1937; Kiricenko and Bassarskaja, 

 1937). 



The adaptability to length of day of various species of the potato has 

 been reported by Driver and Hawkes (1943). Though most wild South 

 American potatoes were found to be short-day plants, certain clones of 

 Solanum andiginum, S. curtilobum and .S". tenuifilamentum appeared day- 

 neutral, and one clone in each of the first two species reacted as long-day 

 plants. 



From these and other instances it is quite apparent that the present 

 classification of plants on the basis of reactions of some members of the 

 group, as regards sexual reproduction, to more or less definite photoperiods 

 is not very appropriate. 



In this connection there are other aspects of the problem of classifica- 

 tion that should be taken into consideration. Very few species, excepting 

 possibly those in equatorial regions and in the case of some desert, polar or 

 high altitude ephemerals, seem to be adapted throughout their length of 



