EVOLUTIONARY TENDENCIES AMONG GYMNOSPERMS 41 1 



well displayed during the Mesozoic. The Ginkgoales, while widely 

 distributed during the Mesozoic, apparently were never a large group; 

 and this group has continued as a single line into the present flora, and 

 has retained certain features of the Cordaitales which the Coniferales 

 have lost. The Coniferales, on the other hand, began that extensive 

 differentiation during the Mesozoic which has resulted in six recog- 

 nized tribes in our present flora. Among these tribes the earliest to 

 be recognized are the Abietineac and the Araucarineac, and their very 

 early separation is so evident as to raise the question whether they 

 may not be independent in origin. In any event, the other tribes 

 recognized in our present flora were of later origin; the Taxodineae 

 and the Cupressineae, and possibly the Taxineae, arising from the 

 mesozoic Abietineae; and the Podocarpineae possibly arising from 

 the mesozoic Araucarineac. 



The connections of the Gnetales are altogether obscure, and every 

 opinion as to their origin must be regarded as very tentative. Although 

 they have not been discovered as fossils, the great amount of 

 differentiation they show and their widely scattered geographical dis- 

 tribution indicate a considerable history. Evidence seems to be 

 accumulating that they may have been derived from the Cupres- 

 sineae, or at least that they are closely related to that tribe in origin. 



VASCULAR ANATOMY 



The central cylinder of the Cycadofilicales, like that of ferns, was 

 protostelic, siphonostelic, or polystelic. Whatever may be the genetic 

 connection of these types of cylinder, the siphonostele is the type that 

 was carried forward in the evolution of gymnosperms. This sipho- 

 nostele was made up of collateral mesarch bundles, developed second- 

 ary wood composed of tracheids, and the bundles of all its peripheral 

 connections were concentric or at least mesarch. 



Among the gymnosperms the universal tendency was to eliminate 

 the centripetal xylem, a tendency carried forward from the ferns, until 

 the collateral mesarch bundles of the central cylinder became col- 

 lateral endarch; and more gradually the bundles of the peripheral 

 regions became collateral mesarch and finally collateral endarch. 

 So early was this accomplished for the central cylinder that a col- 

 lateral endarch cylinder is a feature of gymnosperms in general. 



