Vernalization and Photoperiodism — 106- — A Symposium 



The consensus of opinion of the leading Paleobotanists appears to be 

 that the woody plants are the older of the two groups. However, one can- 

 not be far wrong if he takes the more conservative view that both groups 

 were in existence in Cretaceous times, but that the warmer and more uni- 

 form climate gave the former their high dominance as it does in the tropics 

 and subtropics of the world today. It seems to be a well established fact 

 that the woody Dicotyledonous Angiosperms at the present time are over- 

 whelmingly predominant in the tropical flora, whereas in this class the 

 herbaceous forms have preponderance in all the cooler regions of the world, 

 more especially in the north temperate regions as well as in alpine and arctic 

 lands everywhere. 



It seems to be a somewhat dogmatic point of view to hold that the cold 

 has somehow suddenly (speaking geologically) modified a woody stem, and 

 reducfed its structure by breaking the continuity of the woody ring into the 

 many-bundled herbaceous type. There is probably no botanist who would 

 hold dogmatically that herbaceous plants were non-existent in the Cretaceous 

 flora or in that great interim of which there is no record, that must have 

 prevailed between the laying down of the Cretaceous rocks and the deposi- 

 tion of the tertiary sediments. 



It is not necessary to explain the actual origin of the woody or herba- 

 ceous elements of the ancient flora. It is enough for the present discussion 

 merely to assume that herbaceous members existed in the Cretaceous Period, 

 as most botanists would concede. Although the latter element may have 

 been reduced to a very subordinate status under the warm conditions favor- 

 ing woody forests, there is reason to believe that herbs were in existence in 

 favorable though restricted habitats. 



Evidence has been derived, also from other sources, one source deriving 

 its conclusions from comparative studies of various families with ancient 

 and modern representatives, and noting the relative abundance of the woody 

 and the herbaceous life forms in these. In general the more ancient groups 

 have a preponderance of woody types, while the more modern families con- 

 tain the greatest number of herbaceous forms. While this evidence is not 

 wholly acceptable to some students, others regard these comparisons as of 

 some significance. 



It is indicated, then, that the Cretaceous flora was becoming rapidly 

 Angiospermous on a woody life form, and that this flora was of world-wide 

 distribution, in response to a relatively warm and uniform climate prevail- 

 ing over all the earth, even at the Poles. There was much intermingling 

 of very unlike forms, involving types which are now dissociated and con- 

 fined to distinct climatic and geographic zones. Palms and Poplars, Figs, 

 Breadfruit, and Magnolias appear to have grown together for some un- 

 explained reason, an association of species that is hard to visualize on the 

 basis of our present climatic associations of such plants. However, if there 

 was marked uniformity of climate over all the earth, sufficient to account 

 for these striking interminglings and distributions of a warmth-demanding 

 plant life, it is not illogical to assume that all the dominant factors of the 

 climatic complex had undergone major modifications. It is fair to assume 

 that a uniform length of day of about 12 hours may have accompanied this 

 climatic expression, for such uniformity in daily light duration would be- 

 come a potent factor in forming a widespread or even world-wide distribu- 



