i66 GYMNOSPERMS 



them were tall trees reaching 30 meters in height, and growing in dense 

 stands. Although the trunks sometimes reached a diameter of nearly 

 a meter, and often two-thirds that size, the great height made them 

 appear slender, an appearance accentuated by the branching which 

 was confined to the top of the plant. The leaves range from tw'O 

 centimeters to a meter in length, and are as uniformly simple as the 

 leaves of the Cycadophytes are compound. While some of the large 

 leaves are 20 cm. in breadth, the prevailing type is uniformly rather 

 narrowly lanceolate. The top of the plant must have looked some- 

 what like the modern Screw Pine, Pandanus. Grand d'Eury's 

 restoration, as modified by Scott,^" is shown in fig. 178. 



DISTRIBUTION 



The Cordaitales were most abundant in the Carboniferous, but 

 how far below and how far above that horizon they may extend 

 depends upon the diagnosis of araucarioid stems in the pre-Carbonif- 

 erous deposits, and of leaves in the Triassic and Rhaetic. Krau- 

 SEL^^^ thinks that these lower horizon stems and higher horizon 

 leaves cannot be assigned to this group with much confidence. 

 WiELAND thinks they go back to the Silurian or earlier, developing 

 into great forests in the Devonian, culminating in the great swamp- 

 forests of the Carboniferous, declining in the Permian, and becoming 

 extinct in the Liassic. 



In the Carboniferous, stems, leaves, roots, and reproductive 

 structures are well known, and there has been an assembling of stem, 

 leaf, root, and reproductive structures, as in the famous Lyginopteris. 

 Consequently, in this horizon, isolated parts can be assigned to the 

 group with considerable certainty. Where only the wood is avail- 

 able, as in the lower horizons, it is a matter of individual opinion 

 whether a certain trunk or piece of wood may belong to this group or 

 not. It must be admitted that investigators who have made the 

 most thorough studies of the histological characters of the wood of 

 both living and extinct forms are the ones who lay greatest stress 

 upon the reliability of diagnoses based upon the microscopic struc- 

 ture of wood. The assigning of leaves to the group is also a matter 

 of opinion. When we remember that, on the basis of the leaf alone, 

 it is impossible to determine whether a certain leaf belongs to the 



