4IO GYMNOSPERMS 



The histological structures are so arlvanced that they are respon- 

 sible for an undue amount of theorizing. 



The stem. — A transverse section of a young stem of Gnelum 

 gncmon shows an endarch siphonostcle, with strong bundles, large 

 rays, and a cortex, partly of thin-walled parenchyma and partly of 

 suberizcd fibers, and with a zone of spicular cells just outside the 

 phloem (fig. 383). 



The protoxylem is very scanty, and the markings are spiral. 

 Vessels in the secondary wood are of all sizes, from that of ordinary 

 tracheids up to those with four or five times that diameter. The pit- 

 ting is multiseriate and extends all the way around and on the end 

 walls, which soon break down, so that there are continuous tubes, as 

 in angiosperms. 



The end walls of the cells, which are to form vessels, may be quite 

 oblique or perfectly transverse. There is a single perforation, formed 

 by the enlargement or fusion of bordered pits and the disappearance 

 of the middle lamella — a more advanced type than that of Ephedra, 

 where the end walls have several perforations, and more like that of 

 angiosperms, which have a single perforation. It should be noted 

 that the mode of formation of the more or less freely open tube of 

 Gnelum is not like that in the angiosperms, where the single large 

 perforation of the lower dicotyls has been developed from a scalari- 

 form type. Thompson^^' regards this difference in the development 

 of the vessels as so fundamental that vessels in the secondary wood 

 should not be regarded as any evidence in favor of the theory that 

 angiosperms have come from Gnetales, or that the two have had a 

 common ancestor. 



In many angiosperms the end walls break down while still very 

 thin, and without any connection with pitting, and even in some (in 

 which secondary thickening has begun), the breaking-down does not 

 seem to depend upon pits. 



The phloem consists of very uniform cells. There are no compan- 

 ion cells in Gnetiim gnemon, although such cells occur in G. latifolium 

 and G. scandcns. However, these companion cells are not cut off 

 from a mother-cell, as in angiosperms, but arise independently from 

 the cambium. Here, again, the angiosperm feature, the companion 

 cell, should not be urged as evidence in favor of relationship, be- 



