Allard —103— Length of Day in the Past 



to a measurable degree, but very slowly, so that after many thousands of 

 years it will be reduced from a mean value of about 23° 27' 3", which is 

 near its present inclination, to about 22J/2 degrees. After this it will again 

 slowly increase its inclination. There are many conditions involved in these 

 concepts, and it is doubtful if any categorical statements can be made as to 

 actual happenings hundreds of thousands or even millions of years ago in- 

 volving possible relations of position between earth and sun. It is obvious 

 that even with all our geological evidence of the relatively very recent Ice 

 Sheets, geologists have been unable to explain their occurrences with final- 

 ity or to arrive at any agreement as to why a warm, even sub-tropical climate, 

 devoid of marked zonal and seasonal differences such as prevail today, 

 reigned over much or all the earth in Cretaceous times. 



The earth has existed many hundreds of millions, or perhaps even bil- 

 lions of years since Cambrian time, and stupendous diastrophic events and 

 profound cyclic changes in climate have occurred afifecting the biological 

 trends of life everywhere, but of the actual causes of these inconceivably 

 ancient happenings there can be only supposition. It is possible that the 

 inclination of the earth's axis has not always been as it is today, and that 

 it has even been inclined to a greater or even to a much less degree than our 

 present measurements show. An approach of the obliquity of the earth's 

 axis to zero alone would bring about the most profound changes in the 

 length-of-day aspect of climate and it is probable that new specializations 

 and distributions of plant life would occur over all the earth. While pro- 

 nounced zonations of climate at present obtain, geologists are of the opinion 

 that weak zonations of climate have characterized certain geological eras. 

 It is difficult to harmonize such conditions with a strongly tilted axis such 

 as now prevails. 



The Length-of-Day Factor of Climate and Plant Life: — The 



published results of Garner and Allard in 1920 which for the first time 

 showed that the flowering and fruiting of plants is intimately related to the 

 length-of-day factor, has been amply substantiated by many other scientific 

 workers throughout the world. It is now recognized that plants vary greatly 

 in their responses to length of day. Some flower in response to shortening 

 days, others require lengthening days or days of intermediate length, while 

 a large group may be insensitive to length of day, or show a day-neutral 

 behavior. 



Plants of the short-day type, more especially those flowering in response 

 to lengths of day near 12 hours, under our present terrestrial arrangements 

 of zones and seasonal cycles, are of necessity confined to tropical regions or, 

 if adapted only to the temperate zones, have become autumnal types. The 

 latter flower in response to the shortened days which accompany the ap- 

 proach of autumn as do the native asters and goldenrods. The long-day 

 plants are adapted only to the longer midsummer days, the duration of which 

 increases with latitude until continuous day is experienced for varying pe- 

 riods at or near the poles. The day-neutral plants have no length-of-day 

 limitations, since they can flower in response to all lengths of day prevail- 

 ing from the equator where nearly constant daylight periods of 12 hours 

 prevail, or even survive in high latitudes where very much longer days or 



