94 



THE ANATOMY OF WOODY PLANTS 



gymnosperms and even in the higher representatives of the group 

 as far up as the conifers. It is in the Gnetales, the most advanced 



forms among the naked-seeded Spermo- 

 phyta, that vessels first make their appear- 

 ance in the cylinder of the secondary 

 xylem. Fig. 720 represents a smaller ele- 

 ment of this type from the wood of the 

 genus Ephedra. The walls terminating 

 the structure under discussion are dis- 

 tinctly at angles to the lateral ones and 

 are remarkable for the extremely large 

 pits which cover their surfaces. The pits 

 of the terminal walls are not only very 

 much larger than the lateral pits, but, 

 with two or three exceptions, they have 

 lost their membranes. In the pores where 

 the membranes still persist a well-marked 

 torus reveals its presence in face view. 

 The pits of the lateral walls of the tra- 

 cheids in the case of Ephedra are not very 

 much narrower than those of the terminal 

 surfaces, but invariably are closed by 

 membranes thickened centrally by a well- 

 marked torus. Fig. J2b shows a some- 

 what larger vascular element from the 

 wood of Ephedra, which has its enlarged 

 terminal pit entirely perforate as a result 

 of the complete disappearance of the mem- 

 branes. In Fig. 73 is shown a vessel in 

 which, as an exceptional condition for the 

 genus under discussion, there is a tendency 

 to fusion on the part of the enlarged termi- 

 nal pit. Although in the Gnetales the 

 phenomena of fusion of the end pits of 

 the vessels is rare, it becomes the rule in 

 the angiosperms, as will be shown at a later stage. At a appears 

 a vessel which is transitional from an element of this type to a 



B 



FIG. 72. Smaller and 

 larger vessels of Ephedra. 



