A NEW MEDULLOSEAN STEM. 



97 



The age of Medullosa Solmsii and of other Continental species is Permian (13, p. 48) ; 

 the English species, M. anglica, however, is of Lower Coal Measure age (7, p. 84}. 



In the case of the South African stem, the general appearance and arrangement of the 

 innermost series of steles recalls that of the plate-rings of M. Solmsii, except that in 

 the African type the inverse part is much more strongly developed than the normal 

 part, possibly because towards the centre of the stem there is more space for such 

 development to proceed after the second zone of xylem-masses has begun to develop, 

 cramping the normal parts of the first zone. It has been shown, however, that there 

 is variation in the relative degrees of development of the normal and inverse parts of 

 the rings in M. Solmsii ; while in M. poi^osa, in the case of the outer bundles of the 

 pith {13, p. 70) and sometimes in M. anglica (7, p. 89), there is greater development of 

 xylem tow^ards the central parts of the stem, rurther, in the South African stem the 

 outer plate-rings are not represented, unless the portions of normally orientated xylem 

 at the periphery of the fragment correspond to these, the inverse parts being absent. 

 Or, these masses may belong to an outer zone of normally orientated tissue, such as 

 occurs in M. Solmsii, var. lignosa, although in this form the zone is more or less con- 

 tinuous. The outer plate-rings in this case would be entirely unrepresented. 



The new stem shows agreement with M. Solmsii^ and differs from M. stellata and 

 M. poi^osa, in the extreme reduction or at least non-preservation of partial pith and 

 primary tracheides. There is a marked difference from M. anglica, the steles of which 



show a large primary mass of tracheides and parenchyma (7, p. 88). The isolated 

 irregularly orientated" xylem-groups which occur in the South African stem may be 

 vestigial star-rings, comparable with those very generally present in the central ground- 



mass of Medullosa stems. 



The bundles arising from the inner ring of steles in the new type may be of the 

 same nature as the leaf-traces, which in Medullosa typically arise from the inner plate- 



J 



rinffs. Their ultimate fate, however, cannot be decided, as no cortex is present. 



Secretory canals, which are present in the central ground-tissue of if. Leucharti (13, 

 p. 133) and in the ground-tissue and pericycle of the steles in M. anglica (7, p. 96), 

 have not been distinguished in the South African stem, nor can they with certainty be 

 shown to occur in species of Medullosa other than the two mentioned. 



Sclerotic nests are present in the ground-tissue of the African stem ; according to 

 Weber and Sterzel, however, their presence in the species of Medullosa is uncertain 

 (13, p. 115). Sclerenchymatous bands, isolated in the ground-tissue, are mentioned by 

 Scott for M. anglica (7, p. 96 ; pi. x. fig. 2) ; while for the same species internal 

 periderm is described (7, p. 96; pi. v. figs. 1 & 3; pi. vii. fig. 13; pi. x. figs. 6 & 7), 



a continuous though irregular zone. In the African stem isolated bands 

 of periderm occur apparently without definite purpose. 



Owino- to the variability and wide range of structure shown by Medullosa stems, the 

 only distinct points of difference between them and the South African stem lie in the 

 compact wood of the latter, with its uniseriate medullary rays, and in the pitting of 

 the tracheides. In the new specimen the biseriate pitting of the tracheides resembles 

 that exhibited by the Cordaiteae and Araucariese rather than the multiseriate type 



p2 



occurring m 



