Oliver. — The Ovules of the older Gymnoperms . 473 
palaeozoic type. If this be true, it should be possible to 
realize at every stage in the evolution the factors that have led 
to a modification of the ancestral type or to the persistence of 
some of its characters. In the case of the Cycads this is 
beset with less difficulty. The chief factors suggested above 
were the attachment of the pollen-grains to the wall of the 
pollen-chamber by haustoria and the need for increased space 
for the nutrition of the embryo. In Torreya , however, the 
factors are less evident, though the presence of the enlarging 
embryo midway between the foramina just as the stone is 
hardening (end of July) may not be without significance (cf. 
Fig. 14, el). The retention of the nucellar tracheal sheath as 
a mucilage layer (if it be homologous with the palaeozoic 
mantle) may be correlated with the exiguous nature of the 
water-supply, whilst its possible digestive function may also 
have a bearing. Otherwise the interior of the seed might 
become prematurely isolated from water-supplies. For, being 
completely siphonogamous like the other Taxaceae ( Taxus 
and Cephalotaxus), its retention here cannot be attributed to 
the requirements of spermatozoids. 
As for the other Conifers, they have lost their nucellar 
vascular systems, whilst the pollen-chamber is either quite 
obsolete or represented by the merest pouch. The base of 
the ovule has, however, generally undergone a marked exten- 
sion. 
The problem of the limit of the real ovule in Gymnosperms 
is not a new one. Strasburger made some allusion to the 
question years ago 1 . 
In another place I have emphasized the distinction drawn 
between the original ovule and the phylogenetically younger 
intercalation by proposing the terms Archisperm and Hypo- 
sperm to designate these regions. The phylogenetic history 
of a gymnospermous ovule may be compared to the case of an 
island rising out of the sea which becomes an inhabited centre 
of activity. As the elevation continues the original island 
becomes a remote mountain summit, whilst the newly-won 
1 Die Angiospermen und die Gymnospermen, pp. 124 and 134. 
