Vernalization and Photoperiodism - — 104 — A Symposium 



even continuous light may obtain. Their stature and habitus, however, may 

 be greatly changed by particular lengths of day. 



Plants with intermediate habits of response to length of day, flowering 

 only when the days are neither too short or too long, would be confined en- 

 tirely to temperate latitudes. 



The prevalence of a 12-hour day over all the earth, with accompanying 

 warm temperatures, would favor a poleward extension of the warmer zones 

 with consequent changes in the distribution of tropical plant life. More- 

 over, this tropical length of day, at present localized in the middle zone of 

 the earth, would favor the survival only of the day-neutral forms and such 

 short-day forms as had become adapted to tropical short-day conditions of 

 12 hours, and to the short days of the autumnal season in high latitudes. 

 On a purely length-of-day basis, all long-day plants which had become 

 adapted to midsummer lengths of day exceeding 12 hours, or to the con- 

 tinuous day of the poles, would suffer extinction. 



Anyone who has studied the narrow short-day requirements of the 

 Poinsettia, Euphorbia pulcherrima, which flowers with great difficulty in 

 response to lengths of day shghtly longer than 12 to 12J/2 hours, can readily 

 appreciate the profound changes which plant life as we know it today, would 

 undergo should warm temperatures and a 12-hour day prevail over all the 

 earth. Thousands of other woody and herbaceous plants with a behavior 

 similar to that of the Poinsettia, other conditions being met, would find a 

 means of ready invasion into high northern latitudes, one may presume even 

 to the poles, under certain conditions. It may be stated that a constant 

 length of day coupled with more or less uniformity of temperature condi- 

 tions, would constitute the only climatic environment which would tend to 

 distribute the same flora over all the earth. 



Until the importance of the length-of-day factor was recognized as 

 everywhere operative, in controlling the zonal and seasonal distributions 

 of plant life, temperature control was regarded as the major expression of 

 the climatic complex. There is no other factor of climate, however, that is 

 so fixed and regularly cyclic as length of day. Even with a rigidly constant 

 length of day of 12 hours prevailing over all the earth, such as a vertical 

 axis or other condition would enforce, the temperature relations, even over 

 a generally warm earth would show great local variations. These would 

 be associated with differences in altitude and in cloudiness, differences de- 

 pendent upon depth of the atmospheric envelope traversed by the sun's 

 rays, and many other local effects that operate even at the present time. If 

 zonal differences were evident these would be slight and weakly differen- 

 tiated, and quite unlike the well marked zonal distributions associated with 

 an inclined axis which gives us our seasonal climate today. 



Although the Pleistocene Period was characterized by the development 

 of great continental ice sheets thousands of feet thick in the northern hemi- 

 sphere, a trend toward a seasonal climatic cycle had begun long before this. 

 As a matter of fact some geologists have estimated that at least one million 

 years have elapsed since the first Pleistocene glaciation. Whatever condi- 

 tion led to the great departures from the warm and widespread Cretaceous 

 climate, glaciation was not necessarily a cause but a culmination following 

 changes that had come about in terrestrial and solar relations. 



