On the Tyloses of Rachiopteris corrugata. 83 
less need for this form of closing a wound. Their mode of origin, 
too, upon which Molisch insists, as ingrowths from surrounding 
parenchymatous cells, seems to preclude their formation in Gymno- 
sperms where wood parenchyma is generally absent. The fossil 
wood in which Sir William Thiselton Dyer 1 (’72) has described the 
existence of tyloses is, as he states, evidently that of a dicotyledonous 
tree, and the tyloses fill the well-defined vessels, which are, no doubt, 
in part surrounded by wood-parenchyma. 
In many of the fern-stems and their leaf-stalks, as far as they 
are represented among carboniferous remains, there is apparently 
very little, if any, wood parenchyma present among the tracheids, 
and this fact renders it difficult to explain the presence of tyloses 
in Rachiopteris ins ignis and R. corrugata. 
For though in the rachis figured by Williamson (PI. 16, Fig. 20), 
where the tracheids are only two rows deep, it is possible for 
ingrowths to take place from the surrounding parenchyma on one 
side, yet in some cases there are three rows of tracheids of which 
the innermost is bounded on either side by lignified cells. 
In the stem figured in his earlier paper (’77, PI. 6, Fig. 16), the 
tracheids which are filled with tyloses, are seen to be surrounded on 
all sides with other tracheids, and no wood-parenchyma is visible. 
And indeed that is found to be the case regularly with large groups 
of tracheids in R. corrugata, though some, both on the inside and 
on the outside, abut on parenchymatous cells. One is therefore at 
a loss to understand how in the middle of the woody mass tyloses 
can be formed in a normal manner. 
The supposition that these curious growths may after all be of 
a fungal nature reasserts itself, and a representation such as 
Williamson gives in Fig. 12 of his later paper (’88), where a hypha¬ 
like filament traverses the lumen of the tracheid to become dilated 
into a terminal swelling, recalls forcibly the similar dilatations of 
fungi formed in the tracheids of various lepidodendroid plants. 
In spite of the difficulty of explaining the formation of these 
tyloses, I am inclined to accept Williamson’s identification as 
correct. The occurrence of filamentous outgrowths terminating 
in dilated tyloses may be a necessity where, as in the case of the 
rachis, parenchymatous cells are found only on one side of the 
tracheids. In this case it may be necessary to close the distal 
portion of the lumen by tyloses growing at first as elongate filaments 
between the nearer protrusions until they reach the further side of 
the cell-space, where they dilate into normal tyloses. 
1 \V. T. Thiselton Dyer. On some Fossil wood from the Lower 
Eocene. Geol. Magazine, Vol. XCVI., 1872. 
