XXX] COLPOXYLON 145 



Medullosa for Colpoxylon. The resemblances though close are 

 hardly sufficient to warrant this course. In Colpoxylon the stelar 

 system is simpler ; there is no central region with star- or plate-rings 

 as in Medullosa Leuckarti but, as in Medullosa anglica, the vascular 

 tissue consists only of large steles without a medullary system. 

 Colpoxylon differs from M. anglica in the reduction in some parts, 

 of the stem of the vascular system to a single stele and, moreover,, 

 the primary portion of the steles is much more parenchymatous 

 in structure and contains more irregularly anastomosing tracheal 

 strands than is the case in M. anglica. 



The alteration in the pattern formed by the vascular system 

 at different levels in some Medullosan stems, especially in Colpo- 

 xylon, may be compared with the varying disposition of the 

 vascular strands in the thick dorsiventral rhizomes of Polypodium 

 heracleum Kunz. and P. quercifolium L. In the rhizome of 

 P. heracleum there are two vascular systems, an outer, cortical, 

 system in the form of a hollow cylinder composed of a lattice- work 

 with polygonal meshes from which branches are given off to the 

 roots, and a more complex medullary system that is concerned 

 with the emission of leaf-traces. As shown by a series of drawings 

 reproduced in an account by Klein^ of the anatomy of these 

 species of Polypodium, the inner system of steles consists of two 

 cyhnders connected towards the upper surface of the stem by a 

 rounded arch of vascular strands; nearer the leaf-base the two 

 cylinders meet and eventually a larger cyhnder is produced partly 

 from the upper halves of the two cyhnders of the previous section 

 and in part from the connecting arch: the remains of the two 

 smaller cylinders become connected with the outer vascular system. 

 These and other changes suggest comparison with Colpoxylon as 

 also with the stelar changes in the stem of Ptychoxylon. The 

 comparison cannot be carried beyond the grosser features and is 

 chiefly interesting as affording a further illustration of a similarity 

 in plan between some recent Ferns and extinct Pteridosperms 

 and other Palaeozoic genera. 



1 Klein (81) Pis. xxu. — xxiv. 



10 



