641 
Ovule of Bowenia spectabilis. 
relation of the cells composing this tissue are also extraordinary in some 
cases. In every ovule examined the prothallus had grown abnormally into 
the nucellar tissue above, a feature which was remarked upon in connexion 
with the younger abnormal ovules. In several instances this growth is 
astonishing. In the ovule in Text-fig. 15, the upper part of the prothallus is 
a large, cone-shaped mass, the apex of which has pushed into the pollen- 
chamber. In the ovule in Text-fig. 16, a mass of prothallial tissue has 
grown out through the pollen-chamber to the cavity between nucellus and 
integument. In all these cases the tissue of prothallus and integument is 
very intimately connected. 
The cause of the abnormalities in the two sets of ovules just described 
is probably twofold : first, the unnatural conditions under which the plants 
grew, and secondly, the lack of pollination. The first reason seems to account 
for the abnormal appearance of the younger ovules. It is clear from com- 
paring one of these ovules (Text-fig. 14) with a normal ovule just before 
pollination (Text-fig. 4), that at an early age it must have ceased to follow 
the usual line of development and yet have produced a prothallus and 
pollen-chamber. The influence which caused this departure must have been 
felt before the ovule was normally ready for pollination, for the ovule is 
actually less in size than the younger ovule with no prothallus and pollen- 
chamber. The lack of pollination, however, would probably account for 
the unusual appearance in the older ovules. 
Morphology of the Pollen-chamber. 
From a comparison of the different stages of development of the 
nucellus in Bowenia and other Cycads it seems probable that the pollen- 
chamber arises similarly in all cases. Before pollination the nucellus has 
a central strand of cells differentiated from the cells of the surrounding 
tissue (Fig. 7, PI. LXI). About the time of pollination the apical cells of this 
strand begin to break down. They apparently separate from each other 
and are then completely dissolved. The first result of this ‘ resorption ’ is 
that a small cavity, such as that figured in Stangeria } termed here the 
upper pollen-chamber, is formed in the apex of the ovule. This small 
cavity has walls which are cutinized and taper off to a fine point above. It 
seems probable that pollination would normally take place about the time 
of the formation of this upper cavity, and that it is by reason of the breaking 
down of the cells in that region that the pollen ever reaches the nucellus. 
The passage of a pollen-grain through the micropyle, which has become 
very narrow at the time of pollination, would be very difficult unless there 
were some suction from below ; cf. Webber on Zamia . 2 
In Bowenia , before the pollen-chamber has begun to form, the micro- 
pylar tube contains mucilage, which probably at a rather later stage assists in 
1 Lang, loc. cit., Fig. 15. 2 Webber, loc. cit. 
U U 
