442 STUDIES IN FOSSIL BOTANY 



form an almost continuous ring, with few interruptions. 

 In some cases the peripheral ring appears to have been 

 completely unbroken, forming a solenostele, such as 

 we find in various recent Ferns and Fern-allies, as, for 

 example, in Loxsoma, Dicksonia, and Marsilia. In all 

 cases, however, so far as satisfactory investigations have 

 been made, the structure of the individual stele is 

 essentially like that in Medullosa anglica. The interior 

 is occupied by primary wood, sometimes containing 

 a rather large amount of parenchyma, and this is 

 surrounded on all sides by secondary wood and phloem. 

 The small central steles usually have an approximately 

 circular transverse section ; they anastomose both among 

 themselves and with the peripheral steles, which they 

 resemble in structure. We see, then, that in all these 

 forms of Medullosa, the structure of the stem was 

 polystelic, as in the simpler species, M. anglica, with 

 which we started. The differentiation of the steles 

 into a central and peripheral system marks, however, 

 a distinct advance on the part of the later representa- 

 tives of the genus. The outer, more or less continuous, 

 stelar zone allowed of indefinite growth in thickness, on 

 its free external side. It is the rule, in the older stems 

 of the type of Medullosa stellata, to find that the great 

 bulk of secondary wood and bast was developed on the 

 outer side of the peripheral steles ; on the inner side of 

 the same zone, and around the small central steles, the 

 growth was of necessity limited by want of space. 

 Consequently, an old stem of Medullosa stellata may 

 assume a very different appearance, as seen in transverse 

 section, from that typical of the genus, the external 

 wood and bast predominating so largely over all the 



