298 MORPHOLOGY OF AXGIOSPEKMS 



Fig. 108, E, shows a type of central cylinder which at first 

 sight appears very like that of the adult stem of Pteris aqui- 

 lina ; above on the right is a gap in the tubular stele, which 

 in this case corresponds to a branch. Laterally, on the left, 

 a foliar trace is to be seen in the fundamental tissue. The leaf- 

 trace is very small, and there is no gap in the central cylinder 

 corresponding to it. As in P. aquilina, there are two medullary 

 fibrovascular strands, ft has recently been shown ( Jeffrey 13, 19 ) 

 that in certain great groups of plants foliar gaps are constantly 

 present, while in other great groups they are unfailingly ab- 

 sent. The type of tubular stele characterized by the presence 

 of foliar gaps has been called " phyllosiphonic," and that pos- 

 sessing only gaps fur the branches or ramular lacunae " clado- 

 siphonic." These distinctions are extremely constant, and con- 

 sequently of great phylogenetie value. 



Fig. 108, F, is from a photograph of a cross-section id' the 

 central cylinder of Osmunda Claytoniana. It is of special in- 

 terest because it is obviously of the same type as the central 

 cylinder of the living Gymnosperms, viz., a ring of collateral 

 bundles surrounding a medulla and separated from each other 

 by medullary rays. Van Tieghem 4 regards this type of stele 

 as derived by dilatation from the prostostelic condition, with 

 the formation of pith and medullary rays from the stelar pa- 

 renchyma. According to this view, the pith and rays are mor- 

 phologically different from and have nothing in common with 

 the fundamental tissue surrounding the stele. 



Fig. 109, G, shows the forking of the central cylinder of 

 Osmunda. cinnamomea. In this example the pith is obviously 

 continuous with the external cortex, and a strand of the very 

 characteristic brown sclerenchymatous tissue of the cortex is 

 passing down into the medullary parenchyma through the gap 

 between the divisions of the fork. It is to be noted further 

 that the phloem passes inward around the divisions of the fork 

 for a considerable distance, and the endodermis is as well 

 marked on the inside as on the outside of the erescentic zones 

 of bundles. In Fig. 109, H, there appears a not unusual con- 

 dition of the central cylinder in O. cinnamomea. Unlike 0. 

 Claytoniana, there is present an internal endodermis along the 

 inner margin of the bundles, and the medulla is often charac- 

 terized by the presence of a mass of brown sclerenchvma similar 



