GNETALES 367 



been described in an earlier page of the present work. They 

 possess clearly marked bordered pits larger than those of the 

 dicotyledons and provided with a distinct torus. The vessels 

 of the tribe are of considerable interest from the standpoint of 

 the doctrine of descent, as they clearly indicate the derivation of 

 vascular structures from tracheids. This takes place by the 

 modification of the terminal regions of the incipient vessel from 

 gradually tapering to distinctly inclined walls at angles with the 

 sides of the element. These differentiated terminal aspects of 

 the vessels are provided with very much larger pits than are found 

 in the lateral walls. These pits, however, generally in Ephedra, 

 and apparently universally in the two higher genera, lose their 

 membranes at an early stage, and free intercommunication is 

 thus established. In the higher genera there is a marked tendency 

 for the terminal walls of the vessel to develop a single huge bordered 

 pit in which the membrane is lacking. In Ephedra, on the other 

 hand, the terminal pits are numerous, and in a few cases are 

 found to fuse with one another horizontally with the resultant ap- 

 pearance of slits comparable to those in the vessels of the lower 

 dicotyledons. The type of vessel found in Ephedra has been re- 

 cently stated to persist in the reproductive axes of the genus Gnetum. 

 The parenchymatous structures of the Gnetales need no extended 

 reference, for on the whole they resemble those of the higher Coni- 

 ferales, both in their distribution in the annual ring (and this is 

 diffuse) and in their configuration. Sometimes structures occur in 

 the woods of the Gnetales more or less resembling substitute fibers, 

 since with pointed elongated configuration they unite a persistence 

 of protoplasmic contents. It is clear from the brief summary of 

 the organization of the wood here supplied that the Gnetales are 

 of great importance from the evolutionary standpoint, particularly 

 in connection with the important problems presented by the 

 evolution of large rays and vessels. They furnish a valuable 

 criterion for the estimation of primitive anatomical characteristics 

 in the organization of the wood in that huge heterogeneous aggre- 

 gation of forms assembled under the caption of dicotyledons. 

 Their value in this respect will appear at a later stage. Although 

 the Gnetales clearly indicate conditions of anatomical organization 



