LENGTH OF DAY IN THE CLIMATES 



OF PAST GEOLOGICAL ERAS 



AND ITS POSSIBLE EFFECTS 



UPON CHANGES IN PLANT LIFE 



by 

 H. A. Allard 



Formerly Senior Physiologist, Bureau of Plant Industry, Soils and Agricultural Engineering, 

 Agricultural Research Administration, United States Department of Agriculture 



Introduction: — Only within comparatively recent years has length of day, as a 

 factor of climate affecting the behaviors and natural distributions of plants, been given 

 any emphasis. In all theories purporting to present the effects of the climatic com- 

 plex upon plant life, even with reference to present conditions as well as to past 

 geological eras, the temperature factor has always been given major consideration. The 

 universally operative factor of daily light duration, in many respects equally as im- 

 portant as the temperature factor, has usually been entirely ignored. 



Plant life in one form or another has existed upon the earth for many hundreds of 

 millions of years. If length of day is a potent factor in affecting the various stages 

 of development of plants growing at the present time, it is reasonable to believe that it 

 has been effective throughout all past geological periods. So long as there is light, 

 terrestrial rotation, inclination of the axis to the plane of the ecliptic, and revolution 

 of the earth around the sun, cycles of length of day will continue to operate upon plant 

 and animal life upon the earth. 



Length of day must, therefore, always be a function of every climate. Geologists 

 inform us that there have been great changes in world climate throughout all the great 

 geological eras. Some climates have been characteristically warm and weakly zonal 

 and others have been cool and strongly zonal. That there have been profound local, 

 regional and even world-wide changes in climate involving length of day as well as 

 temperature cannot be denied. 



The astronomical relations responsible for earth climate are very complex and in- 

 volve many factors including the earth's obliquity, the rate of rotation, the distance and 

 eccentricity of the path of revolution around the sun, the length of time required to 

 complete this revolution, as well as various conditions pertaining to the intensity of the 

 solar energy and many physical conditions obtaining upon the earth itself. These 

 astronomical relations of the earth and sun have engaged the attention of the most 

 eminent astronomers and mathematicians. To attempt to determine the conditions of 

 past geological climates and to learn the relations of these climates to the flora of any 

 era on purely theoretical grounds seems a nearly hopeless task. Yet, if complete 

 fossil records were obtainable so that the actual changes in the various geological 

 floras could be properly evaluated, real material evidence would be at hand to supple- 

 ment the astronomer's mathematical concepts. 



It is a matter, then, of great importance to accumulate all available evidence relative 

 to the changes that have taken place in the floras of past geological eras. As a matter 

 of fact there is little other critical surviving proof that certain types of climate have 

 existed in ancient geological periods, or that changes in the two great zonal controlling 

 factors of climate, temperature and length of day have actually occurred and impressed 

 themselves upon plant life. Whatever astronomical causes may have been responsible 

 the character of the fossil remains of an ancient plant life more than any other evi- 

 dence, may furnish the final proof that profound changes have taken place in the climate 

 of a particular geological era. The astronomer, on purely mathematical grounds, may 

 theorize about changes in the obliquity of the axis. However, were this obliquity 

 actually to approach zero, with the same daily rotation that we have at present, caus- 



