412 McNicol. — On Cavity Parenchyma and Tyloses in Ferns. 
vertical. In one longitudinal section of Rachiopteris insignis I noticed that 
in two wide scalariform tracheids adjoining each other, one was filled with 
pseudo-parenchyma, whilst the adjoining one contained none : there was 
not even a beginning of growth of the pseudo-parenchyma, as one might 
have expected, by analogy with cavity-parenchyma formation ; in the latter 
case one vessel does not become entirely filled up with a tissue before 
sending out processes into the adjoining vessels. A long fungal filament ran 
vertically in the otherwise empty tracheid. On the whole, it seems to me 
that there is at least as strong evidence in support of the view that the 
pseudo-parenchyma in the tracheids of fossil Ferns is due to the presence 
of fungi as for the view that it is due to the xylem-sheath cells which fill up 
the lumina of the tracheids by a true process of tylosis. 
To Professor Weiss, although I differ from him in my view as to the 
formation of soft cells in the tracheids of fossil Ferns, I wish to express my 
indebtedness for help in this piece of work and for the useful suggestions he 
has made to me from time to time. 
University of Manchester. 
January , 1908. 
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