22 Oliver and Salisbury. — -On the Structure and 
The state of tension in the plinth tissue, depicted above, would become 
effective as the enzyme-action, which had already dissolved the core of the 
lagenostome, advanced basipetally and involved the tissues of the plinth. 
For the effect of the enzyme would be to soften the central tissue of the 
cushion or lens (Text-fig. 7 B), thus removing serious resistance to the ver- 
tical extension of the plinth cavity. In this way a central pad, adhering to 
the floor, would be likely to separate from the cushion, which in its turn 
would remain hanging to the base of the lagenostome. The separation of 
the flanks of the cushion from the shoulders of the plinth may well have 
been effected at the same time, whilst in occasional examples (like R. 117, 
PI. II, Fig. 16) the whole of the tissue of the cushion may have been liberated 
from the top of the plinth. 
It is of course possible that only the peripheral layers of the core of the 
lagenostome were softened (and not the whole dissolved), so that when the 
enzyme-action had reached the base the whole of the central mass of tissue 
became separated from the lagenostome to form the pad on the floor. This 
would perhaps account for the unusually large dimensions of the pad in 
R. 1 17 (Fig. 15, pdl). For a decision on this point, which is not very 
material, the needful data are wanting. As to this, however, we are 
satisfied : that a hole was drilled which established communication between 
the cavities of the lagenostome and plinth, and that the pollen descended 
through this orifice into the plinth chamber. 
The second stage in the development of the mechanism is reconstructed 
in Text-fig. 7, C. 
Our third stage is represented by specimens R. 119 and R. 123 (Figs. 
11 and 19), in both of which — unlike any of the others — a prothallus 
is present. In both we find the lens in contact with the lagenostome above, 
whilst its lower surface rests on the arched floor. Moreover, in R. 123 
a pollen-grain is present in the plinth cavity, whilst one of the bodies in a 
similar position in R. 119 may possibly be of the same nature, though the 
preservation is not good enough to say definitely. The problem we are 
trying to solve is the presence of the grain in this position with the other 
parts apparently blocking the way (Fig. 19). 
Our conclusion as to the course of the pollen has already been stated ; 
it made its way through a temporary orifice. 
As the seed continued its development the prothallus made its appear- 
ance and a ‘ tent-pole ’ prothallial apex pressed on the septum from below, 
restoring the relations as we see them in specimen R. 123 (Fig. 19). The 
presence of a 4 tent-pole ’ is indicated in both specimens — especially striking 
is it in R. 119 (Fig. 11, t.p.), where the somewhat oblique plane of section 
has cut the prothallus twice, once at the projecting shoulder and again at the 
tip. Williamson’s figure 1 appears to represent a seed in the same phase as 
1 Williamson : loc. cit., PI. XII, Fig. 86. 
