SIZE VAUIATIOX IN' THA( HKARY CF.T.LS. 193 



their lateral secondary walls, but this is not the case in groups III and 

 IV. In other words, with increasing specialization certain tracheary 

 cells become highly mocHfietl and serve principally as conductors of 

 liquids, whereas others gradually cease to ser\e in that capacity, and 

 become modified as mechanical or skeletal elements. Fig. 6. Each 

 line of specialization appears to involve a general tendency towards 

 reduction in the length of the tracheary elements and concomitant 

 changes in the structure and arrangement of the pits. 



Furthermore, as the secondary xylem becomes more and more 

 highly differentiated, the contrast in length — in the later formed 

 growth layers — between vessel-segments and surrounding tracheary 

 elements usually becomes correspondingly accentuated. 



The fact that ^•essel-segments are shorter than the tracheids, fiber- 

 tracheids or libriform fibers which surround them, and that, in the 

 dicotyledons, the tracheary elements become shorter as the vessels 

 become more highly differentiated, suggests that the striking general 

 contrast in cell length between the secondary tracheary elements of 

 the gymnosperms and dicotyledons may be closely correlated with the 

 evolution of vessels in the later group of plants. That the reduced size 

 (length) of the tracheary elements in the dicotyledons is associated in 

 some way with the development of vessels is emphasized, furthermore, 

 by the fact that such vesselless arborescent plants as Trochodendron 

 and Drirays have the longest tracheary elements of any of the dicotyle- 

 dons investigated. Table VII. In addition it is significant that the 

 tracheary elements of the secondary xylem of the Gnetales, supposed 

 gymnosperms with vessels, resemble in length those that occur in 

 many dicotyledons, Table VII. 



TABLE VII. 



Vesselless A ngios-perms . 



Trochodendron aralioides Sieb. & Zucc. Tracheids 

 Drimys Winteri Forst. " 



Gnetales. 



Ephedra calif ornica S. Wats. Vessel-segments 



Tracheids 

 Gnetum Gnemon Linn. Vessel-segments 



Tracheids 

 Welivitschia mirabilis Hook. f. Vessel-segments 



