PALAEONTOLOGY. 453 



studies of growth-ring features, and larger climatic data of ancient 

 forests not based on both the polished transverse trunk and thin sec- 

 tions, can not rise much above uncertainty. Growth-rings or rather 

 growth-ring-like appearances are nearly omnipresent in petrified woods 

 all the way back to the Carboniferous; but such often present great 

 variations and peculiarities; they may appear to the eye as charac- 

 teristic as in a northern oak, and then on examination, under even low 

 powers, the rings may fade away as a mere result of crushing in the 

 opaloid state, or even prove to be more or less discontinuous. 



Many colloidal effects with inequal iron staining and carbon banding 

 must arise during the course of petrification; and just as there is a 

 nodular tendency in rocks to develop spheroidal banding, so in a petri- 

 fied stem concentric cylindrical features often appear which are 

 secondary to the wood structure, or which highly exaggerate (to the 

 unaided eye) the more recondite stem structures and constitution. 

 Thus there is an important banding effect in many stems due to what 

 may be called shadow growth-rings, or "chemical growth-rings" as 

 determined in present-day woods by Brown, while in other instances 

 there are discontinuous zones of wood parenchyma. 



The final value of the climatic indices afforded by the Mesozoic 

 conifer forests like those of the Como, the Lakota, and the much earlier 

 Shinarump of Arizona must remain indeterminate for a time longer, 

 although it is probable that the ancient evergreen stands again and 

 again stretched into climates as cool and changeable as southern 

 Alaska. In both the Como and Lakota stems there are sharply out- 

 lined rings. For instance, in the Lakotan pityoxylons there is a long 

 growth of open wood, followed by a discontinuous zone of parenchyma- 

 surrounded resin canals. Dry-season wood is of slow, long growth, 

 and sometimes an even growth of wood and quite confluent rings of 

 several years indicate more evenly favorable seasons. 



In several shorter notes in "Science," taken in conjunction with dis- 

 cussions of the Mesozoic cycadeoids, a theory of Mesozoic stem evolu- 

 tion is in reality brought out. It is believed that a basal scalariform 

 type is seen in Cycadeoidea and that other old types were more arauca- 

 roid, that tracheids could by relative increase in size give rise to vessels 

 and also decrease to the condition of fiber tracheids as especially held 

 by Bailey, and that ray complication with development of storage 

 tissue came rather late. It is not believed that the scalariform woods 

 are less primitive than pitted woods. But it is clearer that, throughout 

 the Triassic and down into the Cretaceous, the widespread to dominant 

 conifers were of generalized structure. The wood is pine to araucaria- 

 like, and the rays often quite simple. A large part of the ancestry of 

 the recent types is doubtless within view. 



One larger result of these studies is emphasized here. Recent 

 anatomically based speculations of botanists, to the effect that herba- 



