FIBROVASCULAR TISSUES: VESSELS 



97 



ordinarily reveals the face view of the characteristically perforated 

 terminal walls of the vessel. We find that it is distinguished by 

 the presence of open slits which are in general without borders and 

 by smaller or larger clearly bordered pits. The latter structures 

 retain their membranes and, where they become extremely elon- 

 gated, are clearly the result of the fusion of two or more 

 horizontally approximated 

 bordered pits. Fig. 7 5 , which 

 represents part of the termi- 

 nal wall somewhat diagram- 

 matically and on a larger 

 scale, shows the mode of 

 fusion of pits and also shows 

 that the final result of this 

 process, when a number of 

 horizontally seriate pits are 

 concerned, is the formation 

 of an elongated slit which 

 reveals its primitive nature 

 only by the retention of bor- 

 ders at the ends. The slit as 

 a whole not only has lost its 

 borders, but likewise the mem- 

 brane of the row of fused pits 



has disappeared. It is thus evident that the slits which occur 

 between the horizontal bars of the lattice work in the terminal 

 walls of a vessel of this type are the result of the fusion of rows of 

 pits, accompanied by a simultaneous loss of pit membranes. 



Before we leave the subject of the mode of origin of the per- 

 forations which characterize the terminal walls of vessels of the 

 lower type in the angiosperms it will be well to discuss the situa- 

 tion in another group which shares with the true Amentiferae in 

 the minds of students of evolution the claim to be primitive repre- 

 sentatives of the dicotyledons. In Fig. 760 is shown a vessel from 

 the root of Liriodendron Tulipifera. In this instance there is no 

 sharp differentiation between terminal and lateral walls, although 

 in general in the genus such a distinction is clearly present. In 



FIG. 75. Diagram showing the origin 

 of scalariform perforations by the fusion of 

 pits in the end wall of the vessel. 



