412 MORPHOLOGY OF GYMNOSPERMS 



From what has been said as to the variable rate of change among the 

 members of a great group, it would be expected that mesarch and 

 concentric bundles might be found in peripheral parts of certain 

 species or genera whose allies had completely eliminated centripetal 

 xylem, or might occur occasionally in any species or genus. It is 

 an interesting fact that centripetal xylem appears to linger longest 

 in the cotyledons; and the number of gymnosperms in which it is 

 known to occur in this organ, regularly or occasionally, is increasing 

 rapidly. 



In the development of secondary wood, the general tendency among 

 gymnosperms is to increase it in amount, so that a thick vascular 

 cylinder is built up by the primary cambium. This tendency is 

 apparent among the Cordaitales, but it reaches its most conspicuous 

 result in the Coniferales and Ginkgoales. This changed also the 

 general topography of the stem, both pith and cortex being much 

 reduced in relative amount. The Bennettitales and Cycadales 

 responded feebly if at all to this tendency, one of their features being 

 the retention of the general stem structure of the Cycadofilicales. In 

 these groups the primary cambium is either short-lived or functions 

 very slowly, and in some forms secondary cambium produces cortical 

 bundles; but the formation of secondary wood never prevents the 

 formation of a large pith and an extensive cortex. It is in these groups 

 also that the concentric and mesarch bundles are most common in 

 the peripheiral members, being found somewhere in all forms; while 

 among the Coniferales the centripetal xylem has been almost com- 

 pletely eliminated. In vascular anatomy, therefore, the Cycadales 

 have retained more ancient features than any other living group. 



The vascular condition among Gnetales can hardly be spoken 

 of in connection with general tendencies; but the appearance of true 

 vessels associated with the tracheids of the secondary wood is too 

 important to omit. In any event, these true vessels of the secondary 

 wood suggest that in the evolution of the vascular cylinder the original 

 tracheids of the secondary wood are finally and gradually replaced 

 by true vessels. 



THE LEAF 



The leaves of gymnosperms may be used to illustrate a structure 

 that exhibits no general evolutionary tendency, but responds more or 



