4 Lang, Vascular Anaiomy of tJie OpJiioglossacecE. 



stage with a medullated stele is reached, the protoxylem 

 elements being situated at the inner margin of the 

 wood (endarch), {^Fig. IB). All the xylem at this stage is 

 centrifugal but soon metaxj'lem elements appear to the 

 inside of the protoxylem and the mesarch structure is 

 established. This is found in rhizomes of various diameter 

 and is independent of — and long precedes any appearance 



Fig. 2. A. Transverse section of small stele of He!- 

 niiuthostacJiys. The gap in tlie mesarch xylem is closing. 

 The departing leaf-trace is endarch but has completed the 

 centrifugal xylem on the adaxial side. 



B. Leaf-trace farther removed from the stele, showing 

 the " clepsydroid " stage. (X30. ) 



of — an internal endodermis. In small rhizomes the 

 elements of the centripetal xylem may be scattered 

 through the pith {Fig. ic) but they are soon restricted to- 

 a peripheral position just within the protoxylem (^Fig. 2). 

 The stele in strong mature rhizomes of Hebnintho- 

 stachys {Fig. 3) has a large pith around which comes the 

 hollow cylinder of xylem ; phloem is present between 

 the xylem and the external endodermis but there is 



