FIBRO VASCULAR TISSUES: TRACHEIDS AND FIBERS 29 



winter storms of later geologic times. It is unlikely that the trees 

 of the Paleozoic age were exposed to the often furious air currents 

 which characterize the present age of extreme physiographic and 

 climatic differentiation of the surface of our earth. 



It is convenient at this point to introduce a general statement 

 in regard to the organization of the wood of Paleozoic and Meso- 

 zoic forms so far as the tracheary elements of the secondary wood 

 are concerned. In the case of the giant club mosses or lepi- 

 dodendrids of the earlier forests, the secondary growth consisted 

 entirely of tracheids with scalariform markings similar to those 

 characteristic of the primary wood of many forms. These elements 

 were provided with pits on both their radial and their tangential 

 walls, in this respect offering a distinct contrast, as likewise in their 

 sculpture, to the gymnosperms of the same period, which, as has 

 been indicated above, possessed only pitted tracheids in their 

 secondary growth and had the pitting strictly confined to the 

 radial surfaces of the elements. In the arboreal forms included 

 under the general heading of sigillarians the scalariform sculpture 

 often gave place to the pitted condition, but the pores, as in the 

 lepidodendrids, were both radial and tangential in distribution. 

 In the sphenophyllums and their allies, the calamites, the sculpture 

 was transitional from scalariform to pitted and was confined to 

 the radial aspect of the tracheids. In the true gymnosperms of 

 earlier ages, including the Cycadofilicales (Pteridospermae) and 

 their allies, as well as that group of forms included under the con- 

 venient cognomen of Cordaitales, the sculpture of the secondary 

 elements of the wood was pitted (not scalariform) and was con- 

 fined to the radial aspects of the cells. It is thus clear that in the 

 great Paleozoic age the water-conducting elements of the secondary 

 woody growth, so generally prominent in both vascular cryptogams 

 and gymnosperms, were much more uniformly organized than is the 

 case with plants of later geologic times. 



The further course of evolution and differentiation in tracheary 

 or fibrous elements of the secondary wood can be studied to greatest 

 advantage in the angiosperms, which, although they first appeared 

 in the later Mesozoic, did not become the predominant feature 

 of the flora until Tertiary times. We may conveniently begin with 



