FIBROVASCULAR TISSUES: VESSELS 



95 



tracheid. It is distinguished by the fact that its end walls do 

 not make a definite angle with the lateral ones but taper grad- 

 ually. It is further peculiar in the circumstance that its terminal 

 pits are imperforate with one exception and have their mem- 

 branes marked by the presence of a very distinct torus. In c is 

 figured a tangential view of a vessel in Ephedra, showing the 

 enlarged terminal pores in profile and making it clear that in this 

 case, as in Pteris aquilina, the vessels become patent by the dis- 

 appearance of the membranes of the pits. The organization of the 



FIG. 73. Vessels of Ephedra (after Thompson). Explanation in the text 



tracheae or vessels in the two remaining genera of the Gnetales 

 namely, Welwitchia and Gnelum is not essentially different from 

 that described for Ephedra, and consequently need not be considered 

 here. The Gnetales as a whole are distinguished, not only by the 

 presence of vessels in their wood, but by the possession of rays of 

 the angiospermous type, as has been indicated in the preceding 

 chapter. 



The angiosperms are characterized throughout by the presence 

 of vessels, generally of a high type, but in some cases showing clear 

 indications of derivation from tracheids. It is only in certain 

 xerophytic genera among the Magnoliaceae and in certain Cactaceae 

 and Crassulaceae that these t characteristically angiospermous 



