543 
Origin of Medullation in the Ophioglossaceae. 
old and strong plants. Accordingly it is not stated that Van Tieghem’s 
observations of the interruption of continuity of the endodermis were 
erroneous, or that the endodermal connexions are always such as to con¬ 
stitute a complete barrier. What is stated is that such a barrier does occur 
in certain cases, and that still in them a stelar pith is present. Therefore, 
the establishment of the continuity between the cortex outside the endo¬ 
dermis and the pith within the stele is not a necessary condition for the 
formation of a pith in the Ophioglossaceae. Moreover, as the condition 
where the endodermis forms a complete barrier is the state of the young 
plant, the ontogeny indicates that this was the primitive condition in the 
family. 
There remains the question of the real nature of the pith in these 
young plants where it originates without any direct connexion with the 
cortex, the stele being completely shut off by the continuous ring of 
endodermis. It appears to originate partly from the intraxylic parenchyma, 
which masses together to produce it; partly from the conjunctive tissue 
outside the xylem (or xylem-sheath as it is sometimes called), with which 
the intraxylic parenchyma is connected at the foliar gap. The arrange¬ 
ment of the cells as shown in Figs. 7 and 8 may be held to suggest intrusive 
growth of the conjunctive tissue. But the conjunctive parenchyma is itself 
of stelar origin, as is also the intraxylic parenchyma. Thus the pith is in 
fact of intrastelar origin in these young plants, though there is some 
evidence of intrusion of an outer upon an inner stelar region. 
Returning now to the examination of the primary xylem in Botrychium i 
an inspection of Figs. 6 - 9- shows that while there are in all cases signs of 
cambial activity outside the xylem-tract, and though some of the more 
peripheral tracheides may have had a cambial origin, this is certainly not 
the source of the majority of them, for their arrangement is not in radial 
rows. A comparison of these drawings with those of Professor Jeffrey for 
the seedling of B. virginianum (loc. cit., PL IV, Figs. 6a, 63) shows clearly 
the difference. Whereas in that species primary xylem is virtually absent, 
here it is present in considerable quantity. It became accordingly a matter 
of interest to see the condition not merely at the base of the plant, as 
shown in Figs. 6-9, but also towards the apex, in a case where numerous 
leaves had been formed. This is shown in Fig. 11, which represents a 
section just below the apex of B. Lunaria, where the secondary thickening 
is beginning, but does not appear to have produced any tracheides as yet. 
All those here shown are regarded as primary xylem. It is to be noted 
that the tracheides are associated with parenchyma, which constitutes a 
pith at the centre. Some of the primary tracheides shown in Fig. 11 are 
seen to lie isolated in the pith. They may be held to represent vestigial 
remains of a more extensive primary xylem, which is in course of substitu¬ 
tion by a development of secondary wood. 
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