SPHENOPSIDA 109 



carinal canal (4), in which may be seen lignified rings which 

 are all that remain after the dissolution of annular tracheids. 

 To the outside of each carinal canal, and on the same radius, 

 lies an area of phloem, flanked on either side by a lateral 

 xylem area. This lateral xylem may contain further proto- 

 xylem tracheids with annular thickenings, but otherwise 

 consists of metaxylem elements which may be tracheids 

 with helical thickening, or with pits, or may even be true 

 vessels. Two types of vessel element occur, one with simple 

 perforation plates and the other with reticulate, but it must 

 be emphasized that they are restricted to the internodes and 

 that they seldom occur more than three in a row. They do 

 not, therefore, form conducting channels of great length as 

 do the vessels of flowering plants. ^^ In some species (e.g. 

 Equisetum Utorale) each internodal bundle is surrounded by 

 its own separate endodermis, in others (e.g. E. palustre) 

 there is a single endodermis running round the stem outside 

 all the bundles, while in yet other species (e.g. E. sylvaticum. 

 Fig. 16Q) there are two endodermes, one outside and the 

 other inside all the bundles. 



At the nodes (Fig. 16O) the vascular bundles (i) are 

 connected by a continuous cylinder of xylem, from which the 

 leaf traces (2) and branch traces (3) have their origin. 

 Neither vallecular canals nor carinal canals are present in 

 this region and there have been disagreements as to whether 

 there is any protoxylem here either, but the most recent 

 investigations confirm its presence as a constant feature. ^^ 

 This disposes of the view, held by some, that the internodal 

 bundles represent leaf traces extending down through the 

 internode to the node below. An alternative view used to be 

 held— that the vascular network represents a kind of dictyo- 

 stele, in which the spaces between the internodal bundles 

 represent leaf-gaps. However, this is unhkely, in view of the 

 arrangement known to have existed in the earhest relatives 

 of the genus, such as Asterocalamites, where there was no 

 alternation at the nodes. Furthermore, this view overlooks 



