370 PROF. F. W. OLIVER ON THE STRUCTURE 
entire harmony with such a view. Whether the development of this space (or supra- 
archegonial gap) occurred naturally in the living seed, or whether it only occurred on 
the death of the seed previous to mineralisation, will be discussed below. 
The true pollen-chamber is flanked by a considerable wall of nucellar tissue (PI. 41. 
fig. 2, sh.) which is sometimes admirably preserved. The cells of this lateral cushion 
or shoulder are spindle-shaped, thin-walled, and unpitted, their long axes parallel to the 
slope of the wall (Pl. 42. fig. 11). The surface of this cushion abutting upon the pollen- 
chamber is always ragged and its condition indicates that the deeper-lying tissues have 
broken down. ‘The roof of the chamber consists of a stratum of collapsed and blackened 
cells in which it is difficult to make out any structure. The degree of preservation 
of the shoulder varies from seed to seed. Sometimes, as in fig. 5, the ill-preserved 
remains of a tissue are seen, whilst occasionally (Pl. 41. fig. 1 A) maceration has proceeded 
much further. In sections which are cut at some distance from the median plane of the 
seed, 7. e. tangential sections of the pollen-chamber, the shoulder appears to reach further 
into the pollen-chamber than is really the case (Pl. 42. fig. 12). Between such sections 
and cases like that represented in Pl. 42. fig. 13 a distinction must be drawn. In the 
latter, which, though slightly tangential, is not really very far from the median plane, the 
pollen-chamber is aetually very small, and a great deal of nucellar tissue remains, not only 
at the sides, but below the chamber as well. This seems to indicate that we have here 
a somewhat younger stage in development. The supra-archegonial gap (s.a.g.), too, has 
less vertical development than in the majority of cases, whilst the splitting in the tissues 
to whieh the gap is due does not appear to have occurred quite normally. "The plane of 
eleavage appears to have developed nearer the macrospore-membrane than usual, for 
(1) fragments of this membrane have remained behind on the nucellar tissue; (2) the 
cleft at the lateral angles is nearer the macrospore than we usually find, 7. e. at the 
angles (it is the same on the right as on the leít) the plane of cleavage lies below the 
tracheal sheath, instead of along it (cf. Pl. 42. fig. 13, x x show the plane of cleavage at 
two spots). | 
Comparing the manner of separation of the macrospore from the floor of the pollen- 
chamber in younger and older specimens, it appears that whilst in the usual later stages, 
which form the great majority of those found, the tracheal mantle itself was the fissile 
layer, in the younger stage (Pl. 42. fig. 13) there is a relatively greater cohesion between 
the tracheides than is the case later on. ‘The macrospore in contracting and forming the 
supra-archegonial gap in these younger stages breaks away from the tracheal sheath 
altogether. From this it may be inferred that as development proceeded the tracheal 
mantle became more fissile, 2. e. its elements in this region probably underwent a gradual 
solution prior to fertilisation. When, however, the macrospore separates locally from 
the nucellar wall along the sides or below, the fissile layer is not the tracheal mantle but ` 
the zone immediately within the mantle—precisely as in the younger stage (fig. 13). 
The normal condition of preservation is perhaps a result of the splitting of the tracheal 
layer in this softened condition. The solution of the tracheides had as a probable result 
the promotion of fertilisation. "The actual production of a supra-archegonial gap may be 
regarded as a post-mortem change. 
