20 Oliver and Salisbury . — On the Structure and 
was preserved in the form of a delicate tissue, is not as a rule high enough 
to fill the space between the tapetal septum below and the base of the 
lagenostome above. 
As regards the position occupied by the lens we find two extreme 
states, (a) In some cases the lens is found attached by its convex upper 
surface to the under side of the saucer in which the lagenostome rested 
(specimen R. no, PI. II, Fig. 14, Is.), whilst its periphery hangs down into the 
plinth cavity — its edge being continued as a mere membrane which, often 
interrupted, can be traced obliquely downwards in the direction of the angle 
between the floor and sides of the plinth cavity, where it loses itself in the 
other tissues of the nucellus. Sometimes this membrane runs into the 
plinth wall just above the angle, sometimes it descends more steeply and 
strikes the floor just within the edge. 
In addition to the lens, traces of plinth tissue are also found resting on 
the centre of the floor vertically below the lagenostome in the form of a 
hemispherical pad (R. no, PI. II, Fig. 14 t pd mj and R. in, PL I, Fig. 10 ,pd.), 
the constituent cells of which usually show very poor preservation. The 
relation of this pad to the lens above suggests that it has been derived and 
separated from the middle concave part of the lens as a result of an increase 
in the height of the plinth cavity, with which extension the tissue which 
doubtless originally filled it has not kept pace (Text-fig. 7, C). 
(b) An example of the other extreme state is afforded by specimen R. 117 
(PL II, Fig. 16), in which the whole of the plinth tissue {pl.t) is found resting 
on the level floor of the plinth cavity. In this preparation what appear to 
be the same two portions of tissue are recognizable, viz. the lens tissue 
{pl.t., Fig. 15) with good preservation of its cells, and the central projecting 
boss or pad {pd) around which the former has collapsed. 
These two states are connected by intermediate conditions. Whilst in 
state {a) the convex summit of the lens is in position and still adheres to the 
base of the lagenostome, specimen R. 111 (PL I, Fig. 7, Is) shows some 
slight separation which is a good deal more evident in specimen R. 116 
(PL I, Fig. 6, Is). 
In addition to these cases, which include the majority of specimens, 
there are still others in which the parts seem to approximate to their 
original positions (R. 119 and 123, Pl. I, Fig. 11, and PL II, Fig. 19). 
It should be mentioned perhaps that in these two specimens evident 
traces of a prothallus are present in the megaspore chamber (Figs. 11 
and 19, pr), so that it may be conjectured that the stages in question 
are relatively old ones, whilst the presence of a pollen-grain in the closed 
plinth cavity of R. 123 (Fig. 19 ,p.g) is a remarkable fact which requires 
explanation. 
With a view to elucidating the significance of the various states of 
tissue distribution in the plinth cavity we shall now consider whether and 
