98 MEDTJLLOSEAE l<^^- 



tissue. The term 'partial pith' applied to the tissue between 

 the two cyhnders of secondary tissue is misleading: this tissue 

 (fig. 416, D, p) is the primary xylem of the stele and is homologous 

 with the primary portion of the stele of Heterangium and of the 

 steles of M. anglica. In many sections the continuity of the 

 tubular stele is broken. In a section in the British Museum cut 

 from one of Cotta's specimens^, 6 x 3-5 cm. in diameter to the 

 outer edge of the vascular tissue, the cylindrical stele is interrupted 

 at two places. An example of the interrupted type of stele is 

 shown in fig. 416, F, and in fig. 416, H: the latter belongs to a 

 distinct species. The complete type of cyhndrical stele is excep- 

 tional and occurs occasionally at different levels in the stem. 

 An important point is that the frequent breaks in the cylinder are 

 not connected with the exit of leaf-traces and do not, therefore, 

 correspond to the foliar gaps in the solenostele or dictyostele 

 of a Fern. 



The secondary xylem is of the cycadean type (fig. 418, B, D) 

 like that of Heterangium and Lyginopteris and several other stems. 

 Each of the star-rings in the axial region consists of a paren- 

 chymatous core with scattered primary tracheids enclosed by 

 secondary vascular tissue (fig. 418, B). The star-ring shown in 

 fig. 418, B, from a Chemnitz stem illustrates the characteristic 

 cycadean character of the secondary xylem with broad medullary 

 rays : some of the innermost elements are in contact with the 

 primary tracheids. The phloem is rendered conspicuous by the 

 black contents in some of the elements. Both the star-rings and 

 the larger peripheral steles are constructed on the same plan and 

 agree with the steles of M. anglica. The star-rings occasionally 

 branch and anastomose with one another and with the encircling 

 stele. The star-ring in fig. 416, D at a is about to give off a-small 

 strand. 



Leaf-traces are furnished by the primary xylem at the edge 

 of the ' partial pith ' of the outer stele : as a leaf -trace passes 

 outwards through the outer cylinder of secondary xylem the 

 cambium invests it with secondary xylem and phloem, but as it 

 passes through the cortex of the stem it becomes reduced to its. 

 primary elements, and by successive branching gives rise to 



1 No. V. 8093. 



