THE GYMNOSPERMAE : CYCADALES, ETC. 



751 



surrounds the ring of vascular bundles, which are all collateral and endarch 

 (Fig 753). Large and small bundles are grouped into pairs or threes, iwo 

 small bundles form each leaf trace. These bundles each run a course through 

 two internodes and then pass out in pairs into the leaves. There is thus a 



YiG ^^2.— Ephedra nebrodeme. Portion of a 

 shoot to show the whorls of assimilatory 

 branches. Note the general resemblance 

 to the habit of Equisetiim. 



double leaf trace, in spite of the rudimentary character of the leaves, and we 

 can hardly avoid concluding that this doubled trace is really an ancestral 

 character, pointing backwards in time to the double trace of the Ptertdo- 



'''"rcambium is present and older stems become greatly thickened by 

 secondary growth (Fig. 754)- Annual rings are formed in the secondary 

 wood as growth proceeds, but Ephedra >s not a very long-lned p ant and 

 half a century probably represents its maximum durat.om Medullary rays 

 in the young stems are uniseria.e, but in older stems they become very broad 

 and long vertically, partly bv their own growth and partly by fusion of 

 TefghZ'ring rays7'Their'celis are all Hgn.fied, so that the wood textures 

 veiT hard. The tracheids have single rows ot scattered bordered p,ts, both 

 on the radial and tangential walls, .\lternating double rows also occur. 

 Rims of Sanio and trabeculae across the tracheids are also present. 



