8 4 
F. E. Weiss . 
How the tyloses are formed when the tracheid is surrounded 
on all sides by other tracheids, as in Fig. 12 of this paper, is more 
difficult to imagine. It is, of course, conceivable that a tylose 
might push onwards from tracheid to tracheid, constricting when it 
came to a pit membrane, but I hardly think that can be their course of 
development. In a very thin section of Rachiopteris iiisignis in the 
Manchester Collection there occur in the angles where three 
tracheids meet small spaces which may have been occupied by a 
minute parenchymatous cell, like the small cells met with in a similar 
position in Sphenophyllum. From the absence of walls, however, 
Rachiopteris could only have possessed one such cell, the walls 
of which must have been adjacent to those of the tracheid. 
Occasionally one finds the two adjacent walls of two tracheids split 
apart, and it is quite conceivable that in some cases, at all events, 
there may have been narrow parenchymatous cells between two 
M 
Fig. 13. 
Fig. 12. Portion of the lignified tissue of Rachiopteris corntgata (R447) 
shewing ordinary tyloses and an anomalous type. 
Fig. 13. Anomalous tylose in angle of tracheid enlarged to show 
pitted wall. 
tracheids, which may have been rendered unrecognisable by com¬ 
pression previous to, or since, fossilisation. In one instance, at all 
events, I have met with an appearance which might be explained by 
the existence of such delicate parenchymatous cells between what 
appeared to be adjoining tracheids. The general aspect of this 
preparation, however, suggests that these appearances may be due 
to decay having set in before the stem became fossilized, and the 
separation of the tracheids may be due to partial decomposition. 
A strong argument in favour of the view that these intrusions 
are really of the nature of tyloses is the occurrence in one of the 
sections in the Manchester Museum (R 447) of an intracellular 
projection of somewhat unusual appearance, and closely resembling 
some peculiar tyloses which Miss Rose Jordan described some two- 
