70 THE GEOLOGICAL HISTORY OF PLANTS. 



the original wall uncovered. But this may be done in a 

 great variety of ways. 



The most ancient of these contrivances, and one still 

 continued in the world of plants, is that of the barred 

 or scalarif orm vessel. This may be either square or hex- 

 agonal, so as to admit of being packed without leaving 

 vacancies. It is strengthened by a thick bar of ligneous 

 matter up each angle, and these are connected by cross- 

 bars so as to form a framework resembling several ladders 

 fastened together. Hence the name scalariform, or lad- 

 der-like. Now, in a modern Lycopod there is a central 

 axis of such barred vessels associated with simpler fibres 

 or elongated cells. Even in Sphenophyllum and Psilo- 

 phyton, already referred to as allied to Ehizocarps,* there 

 is such a central axis, and in the former rigidity is given 

 to this by the vascular and woody elements being ar- 

 ranged in the form of a three-sided prism or three-rayed 

 star. But such arrangements would not suffice for a tree, 

 and hence .in the arboreal Lycopods of the Brian age a 

 more complex structure is introduced. The barred ves- 

 sels were expanded in the first instance into a hollow 

 cylinder filled in with pith or cellular tissue, and the 

 outer rind was strengthened with greatly thickened cells. 

 But even this was not sufficient, and in the older stems 

 wedge-shaped bundles of barred tissue were run out from 

 the interior, forming an external woody cylinder, and in- 

 side of the rind were placed bundles of tough tast fibres. 

 Thus, a stem was constructed having pith, wood, and 

 bark, and capable of additions to the exterior of the 

 woody wedges by a true exogenous growth. The plan is, 

 in short, the same with that of the stems of the exogenous 

 trees of modern times, except that the tissues employed 

 are less complicated. The structures of these remarkable 



* First noticed by the author, " Journal of Geological Society," 1865 ; 

 but more completely by Eenault, " Comptes Rendus," IS'ZO. 



