THE PLANT BODY 49 



pitted tracheids. Such an origin makes impossible the derivation of angio- 

 sperms from the Gnetales (sensu lato) or from other higher gymno- 

 sperms that have tracheids with only round-bordered pits. The evolution 

 of the vessel provides strong evidence that the angiosperms arose from 

 fernlike stock. (The resemblance of the vessels of Eupomatia to those 

 of Ptcridium is remarkable.) 



Evidence from vessels supports the theory that dicotyledonous herbs 

 arose from woody ancestors; the vessels of herbs are more highly spe- 

 cialized than those of trees. The obviously recent origin of vessels in 

 monocotyledonous herbs makes impossible the origin of monocotyledons 

 from dicotyledonous herbs, as expressed in some theories of the rela- 

 tionship between monocotyledons and dicotyledons. 



The primitive pitting in intertracheary walls is scalariform-bordered. 

 From this primitive pitting has been derived, by reduction in size, by 

 division, and by change in form, the circular-bordered pit. Division of 

 the elongate pit formed a row of shorter rectangular pits; these later 

 became rounded. In a few of the woody dicot)'ledons, as in Eiipomatia, 

 all the interhacheary pits are scalariform; in others, as in the Win- 

 teraceae, scalariform pits are present only in the early wood, with transi- 

 tional types and round pits in the late wood. The round pits, at first 

 arranged in transverse rows — opposite — become, in higher types, spirally 

 or irregularly arranged — alternate. In the various types of fibers — which 

 are derivatives of tracheids — the pits are modifications of the round type, 

 arranged irregularly. 



In wood fibers of various types — structural modifications of the primi- 

 tive tracheids — the ti'acheid has been reduced in diameter and in num- 

 ber and size of its pits, and its wall has been much thickened. No 

 evidence of the primitive scalariform pits persists; all the pits are 

 reduced forms of the circular pit. In length, there is little change. (The 

 fibers of advanced types of wood appear unusually long in contrast with 

 the shortened vessel elements and the great increase in length in 

 development from the short cambium cell of this wood.) 



Several types of wood fibers are recognized, distinguished on the 

 basis of degree of modification from the ancestral tracheid. Cells in 

 which the wall is greatly thickened and the lumen nearly occluded, 

 with pits small and reduced in number, are typical fibers. Cells inter- 

 mediate between tracheids and fibers in thickness of wall and size and 

 number of pits — the pits with narrower borders and slitlike apertures — 

 are fiber tracheids. Libriform fibers are fibers in which the walls are 

 very thick and the pits very small, often essentially simple. No line 

 separates these types, which represent stages in the elaboration of 

 tracheids as supporting cells, with the conducting function lost. Sub- 

 stitute fibers are fiberlike cells with protoplasts. That they are tracheids 



