506 STUDIES IN FOSSIL BOTANY 



structure of which is in many respects comparable to 

 that of the plants under consideration. Eight internodes 

 below the node the centripetal xylem also disappears, 

 leaving only the wedge of secondary wood to mark 

 the position of the trace (see Fig. 1 8 1 , A, bundles 

 10-14; cf. Calamopitys Beinertiana, p. 482). The com- 

 munication between the different leaf-traces was kept 

 up by lateral fusions, during their passage down the 

 stem. 



The secondary wood presents no peculiarities ; it 

 consists of regular, radial series of tracheides, with 

 medullary rays between them. The rays are of 

 considerable height, and two or three cells in thickness ; 

 the tracheides have several rows of round or hexagonal 

 bordered pits on their radial walls. The structure, in 

 fact, is identical with that of the wood in Lyginodendron, 

 so that the two could scarcely be distinguished. 



The secondary phloem, which is extraordinarily 

 well preserved, is traversed by the medullary rays, and 

 made up of alternate tangential bands of sieve-tubes and 

 parenchyma. Poroxylon is one of the few fossil plants 

 in which the structure of the sieve-tubes can be made 

 out. A radial section of P. Edwardsii is figured by 

 MM. Bertrand and Renault, in which the numerous 

 compound sieve-plates on the radial walls are perfectly 

 plain, just as in some recent Cycads. Heterangium 

 tiliaeoides, Medullosa anglica, and Stauropteris oldhamia, 

 among British fossil plants, sometimes rival Poroxylon 

 in the perfection with which the phloem is preserved. 



The French authors state that there is no pericycle, 

 distinct from the primary phloem. That is a matter of 

 interpretation, but it is an interesting point that the 



