A NEW MEDULLOSEAIS" STEM. 



89 



varying in size and form. In this central system is included the irregularly arched 

 group of vascular elements mentioned above, the group as a whole consisting of two 

 distinct parts separated by a thin line of ground-tissue. In the description of the 

 individual masses of vascular tissue this will be referred to as the " central stele." 



PI. 11. fig. 7, a photograph of Section B from the opposite end of the block, shows 

 the steles lettered and numbered as in PI. 11. fig. 8 and PI. 10. fig. 1. 



Structure of the Ground-tissue. 



Silicification has rendered the material so homogeneous and. transparent that it is 

 impossible to determine definitely the structure of the ground-tissue in which the steles 

 are embedded. It seems, hoAvever, to have been composed of moderately large paren- 

 chymatons cells, not generally very compactly arranged, though the apparent loose 

 structure is probably to some extent due to the shrinking together of the cells and the 

 consequent production of large spaces. A similar process is described by Weber and 

 Sterzel for Medullosa (13, p. 74^). As before mentioned, the ground-tissue at the 

 periphery of the block appears more porous than at the centre. This may be due to 

 more marked shrinking and destruction of the cells in the outer tissues. 



In the curve of the central " stele " is an irregularly oval mass of fairly compact 

 ground-tissue surrounded by a band of varying width of what appears to be periderm 

 (PI. 10. figs. 1, 8, 9, & 11 ; PI. 11. figs. 7 & 8). Here and there the cells of the band 

 are seen to be tabular in transverse section and radially arranged (PI. 11. fig. 6). The 



mass of tissue enclosed by the band of periderm varies in area at different levels 



according apparently to the variation of the curve of the central stele [cf. PI. 11. figs. 7 

 & 8 ; also PI. 10. figs. 1, 8, 9, & 11). Other more or less circular or irregular bands 

 occur here and there, as shown in PI. 11. figs. 7 & 8, pd. 



In Section B the band of periderm encloses a mass of ground-tissue containing 

 scattered and erratic xylem elements ; some of these are cut obliquely, some transversely, 

 but the majority are seen in longitudinal section (PL 10. fig. 11, xy''). The band of 

 periderm itself encloses here and there patches of xylem elements transversely cut. 

 There is no trace of accompanying phloem (PL 11. fig. 6, X2f). A similar enclosure of 

 xylem elements by periderm is figured by Dr. de Fraine (i, p. 1050, text-fig. 16) in the 

 case of two leaf-trace bundles in a stem of SutcliJJia, The author suggests that the 

 abnormal condition may be due to injury perhaps by fungal growth. It is difficult to 

 account for the formation of xylem enclosed by periderm in the present case. In the 

 parenchymatous ground-mass are occasional groups of sclerotic cells, these occurring 

 particularly in the neighbourhood of the steles (PL 10. figs. 3, 7, 10, & 16 ; PL 11. 

 fig. 4). There is no certain evidence as to the presence of secretory ceUs or gum-canals, 

 afthough here and there are cells with dark contents which may possibly represent 



secretory elements. 



Brieflv, then, the ground-tissue consists, as far as may be determined, of large-celled 



parenchyma, with sclerotic nests and bands of periderm in the neighbourhood of some 



of the steles 



2 



