Vernalization and Photoperiodism — 102 — A Symposium 



ing a uniform length of day of 12 hours to prevail over all the earth, accompanied by 

 weak zonations of temperature, there is every reason to believe that profound changes 

 would take place in the floristic life-form and vegetation of the earth. 



Various hypotheses have been advanced to explain the occurrence of warm and 

 cool periods of climate, and the causes of the great Glacial periods. Some of these 

 invoke astronomical changes as in the case of Croll's (1875, 1889) semi-astronomical 

 hypothesis, which makes use of both the inclination of the earth's axis to the plane of 

 its orbit, and an increasing or decreasing eccentricity of this orbit. Others have in- 

 voked changes in the constituents and density of the earth's atmosphere, elevation of 

 land areas known as the hypsometric Hypotheses, actual shifting of the Polar regions 

 due to a thin crust sliding upon a semi-liquid nucleus (A. Wegener, 1920; Koppen, 

 Wladimir Peter, and Wegener, A., 1924), or changes in the localization of the per- 

 manent atmospheric lows and highs. 



Reversals of deep-sea circulation causing the warmer waters to become deep-flowing 

 currents moving toward the poles, while the cold water would flow equatorward, have 

 also been suggested to account for climatic variations. It has been held that such re- 

 versal of the warm and cold ocean currents would explain the tropical and equatorial 

 distribution of the Permo-carboniferous glaciation. 



JoLV (1924) has put forward a theory that the earth's crust periodically warms and 

 melts due to heat liberated by the transformations of radio-active elements. It is sug- 

 gested that this blanketed-in heat would finally cause melting of the basaltic sub-crust, 

 which would yield to various stresses and produce a period of marked crustal deforma- 

 tion or volcanic outflow. While it is generally recognized that the climate of the 

 world has changed very profoundly many times, the causes of the changes are un- 

 known, and no one theory appears wholly satisfactory in explaining all conditions. 



While all of these theories assume that there would be changes in the temperature 

 factor of the climatic complex, only two among these would at the same time involve 

 accompanying changes in the length of day factor. In the one instance change of 

 climate would be dependent upon changes in the inclination of the earth's axis and 

 perhaps changes in the ellipticity of the earth's orbit, with possible changes in rate of 

 revolution, rotation, etc. In the other instance, Wegener's theory of crustal shifting 

 involves changes in the geographical position of the poles. Such events could not occur 

 without causing attendant changes in the daily light periodicity as well as in zonal 

 temperature relations of large areas of the earth's surface. 



The writer has given some consideration to certain angles of this problem in the 

 light of a long study of the responses of plants to length of day. Since previous theories 

 have completely overlooked the very important length-of-day aspects of climate, the 

 recognition of this factor amounts to a new approach to the facts at hand and, for this 

 reason, the present discussion has been prepared. 



Ancient Climatic Cycles : — While geologists agree that great changes 

 have occurred in the climate of the earth since the beginnings of life, the 

 actual causes of these have never been determined. The character of the 

 ancient climate can only be presumed by comparative studies of the mate- 

 rial of the sedimentary rocks and the fossil remains which they contain. It 

 is known that the plant life of the great geological eras has changed pro- 

 foundly from time to time, and it is obvious that these changes must have 

 been greatly influenced by shifts in climate due to changes in astronomical 

 relations, crustal deformation or other causes. 



At the present time the earth is nearest the sun in winter, since it is then 

 in the perihelion portion of its orbit. Astronomers are of the opinion that 

 this is not a constant relationship, and that in about 10,5(X) years it will be 

 at the other extreme of its elliptical path, and reach its greatest distance 

 from the sun known as the aphelion position. It would appear that these 

 theories still assume that the inclination of the earth's axis will remain 

 more or less as it is today. Astronomers make the claim that the mean 

 value of the inclination of the earth's axis is progressively changing 



