CLIMATOLOGY OF THE LATE PALEOZOIC 235 



" (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 char- 

 acteristic of warm climates. Examples are cycadalean types in Carboniferous, 

 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 subtropical types in the Upper Cretaceous; and 

 the bread-fruit trees, etc., in the lower Tertiary. 



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

 growth; that is, abundant rainfall, mild or warm temperatures, etc. conditions 

 favorable to rapid growth : 



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

 spaces, indicating rapid growth and abundant moisture, noticeable in the woods 

 found in and with most coal. 



" (b) 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 pendant 

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

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

 lepidophytes of the Carboniferous. 1 



" (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. 



" (s) 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 main- 

 tain a ground- water table of retarded drainage held by the obstructing vegetation. 



" (?) 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 the stems and branches, as in Ulo- 



" l The interpretation by Davis (in conversation with the author) that the so-called "pseudo- 

 xerophytic " feature of swamp and bog plants whose roots extend near the surface and 

 are normal to a wet footing are for the purpose of protection against destructive suf- 

 fering on occasions of drought when the water-level is usually lowered, and that they 

 are therefore really xerophytic, finds abundant support in the paleobotanical criteria 

 offered by the fossil swamps, as described in an earlier section. The pseudo xerophytic 

 characters appear to indicate probable subjection to occasional times of unusual evapo- 

 ration. 



* Potonie, H., Entstehung der Steinkohle, 5th ed., 1910, p. 167. 



