Actinostrobus pyramidalis, Miq. 325 
to each scale, the scales of the next whorl one ovule each, and all the outer 
whorls being completely sterile. There is, however, considerable variation 
in the number of ovules borne in a healthy cone, the minimum and maximum 
being about six and about twelve respectively. 
The young ovule, about the time of pollination (Fig. 3), shows a long 
and widely open micropyle, and a nucellus free from the integument nearly 
to the base of the ovule. The integument is rather thick, and the cells 
which, at a later stage, close the micropyle are clearly defined on its inner 
surface, these having somewhat denser contents and more conspicuous nuclei 
than the other cells. 
Very soon after this, a single cell near the base of the nucellus becomes 
differentiated from its neighbours. It is distinguished by a larger and less 
deeply staining nucleus and denser cytoplasm (Fig. 4). As far as has been 
seen, never more than one such cell is formed, and it becomes at once the 
megaspore mother-cell. This agrees 
closely with the corresponding stage 
in Callitris , where often only one 
megaspore mother-cell is formed ; in 
that genus, however, two or three may 
be sometimes organized, this being 
probably co-ordinated with the occa- 
sional development of two or three 
embryo-sacs. In Actinostrobus there 
is a complete absence, as in Callitris , 
of any tissue specially differentiated 
as a tapetum. It seems clear, in spite 
of differences of opinion on the subject, 
that the so-called { spongy tissue ’ of 
many Conifers is nothing more nor 
less than the sporogenous tissue of 
the megasporangium, generally, if not 
always, in the mother-cell stage. The 
case of Widdringtonia and Tetraclinis (figured as Callitris quadrivalvis by 
Goebel (9)), where the sporogenous cells are absolutely alike until quite 
shortly before the division of the functional mother-cell, scarcely leaves 
room for doubt on this point. Bower (2), with special reference to the 
Cryptogams, has shown how largely sterilization of potentially sporogenous 
tissue has been responsible for differentiation and advance in the plant 
kingdom, and the conception of the spongy tissue of Conifers as potentially 
sporogenous not only co-ordinates more clearly the megasporangium and 
microsporangium, but also indicates better the undoubted connexion between 
the gymnosperm ovule and the megasporangium of the higher Cryptogams, 
where the sterilization of sporogenous tissue is a still more conspicuous 
Text-fig. 2. Transverse section across 
a young female cone, x 25. 
