196 BAILEY AND TUPPER. 



This is in marked contrast to the conditions which prevailed in the 

 stems of many of the lower vascular plants. In those forms — which 

 possessed relatively wide zones of primary wood — the innermost 

 tracheids of the secondary xylem appear to have resembled in size 

 the elements of the primary xylem. It seems probable, therefore, 

 that in the evolution of the higher gymnosperms and dicotyledons, 

 with reduction in the amount of primary xylem and other changes 

 in the innermost portion of the stele, there has been a concomitant 

 shortening of the first formed tracheary elements of the secondary 

 xylem. 



(2) Another tendency towards reduction in the length of the 

 tracheary elements of the secondary xylem appears to be associated 

 with the evolution and differentiation of vessels. The secondary 

 tracheary tissue of the vascular cryptogams and gymnosperms is 

 comparatively simple and composed of cells of a single generalized 

 type, the so-called tracheids. In the evolution of the Gnetales and 

 Dicotyledoneae, specialization or division of labor appears to occur 

 among these cells. Certain vertical series of tracheids become modi- 

 fied and function principally in conducting liquids whereas others be- 

 come highly modified as mechanical or skeletal elements, Fig. 6. 

 That this process of specialization involves a reduction in the length of 

 the tracheary elements, is indicated, not only by the striking general 

 contrasts between the cells of the xylem in the dicotyledons and gym- 

 nosperms or vascular cryptogams, but also by the interesting fact that 

 the vesselless Trochodendraceae and Magnoliaceae have very long 

 tracheary elements, and the Gnetales comparatively short ones, such 

 as occur in many dicotyledons. As the vessels of the secondary 

 xylem become more and more highly difPerentiated their segments 

 gradually lose their resemblance to tracheids. Fig. 6, and tend to 

 become progressively wider and shorter. At the same time, the 

 structure and arrangement of the bordered pits in their lateral walls 

 tends to be considerably modified. Table VI. Furthermore, the sur- 

 rounding tracheary elements tend to shorten and to take on a more 

 fiber-like structure, their pits becoming vestigial by the gradual dis- 

 appearance of the bordering areas of the secondary- walls. During 

 these processes of specialization, the reduction in length of the vessel- 

 segments usually exceeds that which occurs in the remaining tracheary 

 elements. That is to say, there is less contrast in length between 

 vessel-segments and surrounding tracheary elements in Group I than 

 in Group II of the dicotyledons. 



In addition to these striking tendencies toward reduction in the 



