PALEOBOTANICAL EVIDENCE 505 



Cretaceous floras, and even to a remarkable degree in the Upper Cretaceous 

 and Tertiary floras. 



"4. Presence of types known to be adapted to or confined to the warm tem- 

 peratures or moist climatic conditions of the present day, types that though 

 now extinct once lived in association with other types of ascertained tropical 

 or humid habitats, and types whose descendants or nearest surviving relatives 

 are characteristic of warm climates. Examples are cycadalean types in Car- 

 boniferous, Triassic, Jurassic, Cretaceous, and, finally, in the Oligocene, in 

 association, since the Trias, with living tropical and subtropical genera or 

 families; the presence of tree ferns in nearly all periods of coal formation; 

 palms, cinnamon trees, climbing ferns, and many other tropical or subrtopical 

 types in the Upper Cretaceous ; and bread-fruit trees, etc., in the Lower Ter- 

 tiary. 



"5. Structures of the plants themselves. Features showing rapidity of 

 growth — that is, abundant rainfall, mild or warm temperatures, etc. — condi- 

 tions favorable to rapid growth : 



"(a) Very large size of the cells, many with thin walls, and large inter- 

 cellular spaces, indicating rapid growth and abundant moisture, noticeable in 

 the woods found in and with most coal. 



"(&) Large size of fronds and leaves, indicating conditions favorable to 

 growth and, at present, characteristic of moist tropical habitats. 



"(c) Frequency of laciniate or much-dissected, drooping fronds and pendent 

 branches or twigs seemingly adapted to facilitate the run-off of rain, and pro- 

 tection of the stomata in grooves on the under sides of many leaves, as in the 

 Lepidophytes of the Carboniferous. 



"{d) Smoothness of bark, which is often thick, pointing toward warm 

 humid swamps. 



"(e) Absence of growth rings in the woods of the older coal formations, 

 showing climatic conditions favorable to practically uninterrupted growth, 

 and the absence of long dry seasons or winter frost. Such absence of rings, 

 when noted in all the associated types, plainly shows the approximation to 

 equability of climate. 



"(/) Wide occurrence in the Paleozoic coal fields of heterospory, requiring 

 prevalent swamp conditions; and the occurrence of delayed fertilization and 

 of devices for seed flotation. 



"(^) The development of subaerial roots in many of the types. 



"6. A circumstance that may be observed in most coal fields in proof of 

 abundant rainfall at the time of coal formation is the continuity of many 

 coal benches or strata from one hollow or pan over the intervening shoal or 

 sand bar into the next pan or along the slight gradients of the base levels, a 

 circumstance impossible except with sufficient rainfall to saturate the vegetal 

 cover and maintain a ground-water table of retarded drainage held by the 

 obstructing vegetation. 



"7. Two other interesting lines of evidence for the warm climate of the 

 Carboniferous are seen, as pointed out by Potonie, in (a) the development of 

 more flowers and fruits on the lower parts of stems and branches, as in 

 Ulodendron, Sigillaria, and many Calamariae, a characteristic of dense tropical 

 forests at the present time, and (6) the presence in many ferns of Aphlebiae, 

 which today are unknown except in tropical types." 



