84 GYMNOSPERMS 



consists of scalariform tracheids. The secondary xylem consists of 

 tracheids with bordered pits, except in Zamia and Stangeria, which 

 still retain the scalariform tracheid of their remote fern ancestry. In 

 cone axes and in sporophylls the scalariform tracheid is also retained, 

 but probably not without exception. 



The pitting is usually multiseriate, sometimes with as many as 

 four or five rows of pits. Sifton,"? in 1920, investigated pitting in 

 the cycads and discussed the literature. He believes the bordered pit 

 is derived from the scalariform and cites transitions in Dioon spinii- 

 losum as evidence. The pitting at the ends of tracheids is more primi- 



FiG. T^.—Cycas media: transverse section of stem, showing three zones of wood 



tive than that throughout the rest of its length. He also found ter- 

 tiary thickenings on the walls of tracheids, resembling those which 

 are so characteristic of taxads. This subject will be considered more 

 fully when dealing with the Coniferophytes. 



SiFTON claims that bars of Sanio are present in both primary and 

 secondary wood, while Hale^^o claims that they do not occur in 

 cycads. Differences in interpretation, rather than in observation, 

 seem to be responsible for the discrepancy. 



ITie whole subject of markings on cell walls needs investigation, 

 and a thorough cytological study may yield more reliable results than 

 the usual examination of mature structures. Dr. Grace Bark- 

 ley's-'^ work on Trichosanthcs angiiina showed how the spiral thick- 

 ening arises from the alternation of dense protoplasm with small 

 vacuoles and less dense protoplasm with large vacuoles. That the 

 simple pit was in some way connected with vacuoles in the proto- 



