COMPARATIVE ANATOMY OF GYMNOSPERMS 301 



tudinally, the hypoderma is seen to be composed of a tangential 

 network of sclerenchymatous fibers having the meshes filled 

 with parenchyma. 



Medullosa. — Fig. 110, M, reproduces a diagrammatic trans- 

 verse section of the stem of Medullosa anglica. On the outside 

 of the stem can be distinguished the same curious hypoderma 

 which is characteristic of the genus briefly described above. 

 The central cylinder in this case, however, is obviously not pro- 

 tostelic, but polystelic. Each of the large fibrovascular strands 

 is characterized by the presence of a considerable zone of sec- 

 ondary wood, which is indicated in the diagram by radiating 

 lines. There are no sclerifications in the cortex ; but numerous 

 mucilage duets, similar to those of the Marattiaceae and the 

 ('reads, mav be seen in the fundamental tissue, both outside and 

 between the large fibrovascular strands, although their occur- 

 rence in the latter position is not shown in the diagram. 



Very often the arrangement of the bundles in species of 

 Medullosa was much more complex than that appearing in Fig. 

 110, M. It has been shown recently that in ferns with even 

 the most complex arrangement of the bundles in the adult, by 

 following the development it is possible to arrive at the simple 

 stelar tube as a starting-point (Jeffrey 19 ). It is consequently 

 extremely probable that the bundle system of the Medullosae is 

 to be regarded as primitively siphonostelic, like that of Pteris 

 aquilina. 



In Fig. 110, N, is represented a cross-section of a part of 

 the stem of Medullosa Sohnsi. Here are to be seen numerous 

 bundles, some of which are broad and plate-like and others 

 small and rounded in outline. The broader bundles are known 

 as " plate-rings," and the smaller ones as " star-rings." An 

 interesting feature of the outer plate-rings is the fact that the 

 zone of secondary wood on the external face of the bundles is 

 often very much thicker than that on the internal face. This 

 peculiarity is especially well marked in old stems of M. stellata. 



Lyginodendron. — Fig. 110, O, taken from Williamson and 

 Scott, 6 reproduces admirably the general features of structure 

 of the stem Lyginodendron Qldhamium. On the outside is 

 the same curious hypoderma] layer which occurs in Heteran* 

 gium and Medullosa. There is present also a zone of periderm 

 external to the fibrovascular tissues. In the cortex may be seen 



