THE GYMNOSPERMAE : CYCADALES, ETC. 7^5 



a derivation from the fossil Medullosae, where the vascular system may 

 consist of a group of concentric steles. There are no vessels present m the 

 wood. The protoxylem consists of spiral tracheids, but the metaxylem has 

 bordered pits in several alternating rows on the radial vvalls. ^"-^ ■" 

 type known as araucarioid, which is also found in Cordaites. 



This is the 



;ij 



Fig. Ti^.—Cycas revoluta. Transverse 

 section of part of a polyxylic stem 

 showing two of the concentric rings 

 of vascular tissue. 



The medullary rays are at first fairly broad and deep, so that the wood 

 has the same loose character that is seen in the Ptendosperms. The phloem 

 is broad and contains sieve tubes, parenchyma and fibres. 



A striking feature of the stem anatomy is the " girdlmg " of the stem by 

 the leaf traces. Each leaf is supplied by two main trace bundles, which arise 

 by the branching of one trace which springs from the primary vascular 

 cylinder on the opposite side of the stem from the insertion of the leaf, i his 

 trace springs from the inner edge of the primary xylem and passes out ot the 

 ring through a gap formed by the splitting of a vascular bundle. 



