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apparently the development of the endosperm is not affected. But even 
now, the number of Angiosperm-sacs investigated is comparatively small^ 
and the facts are insufficient to establish any general conclusions regarding 
the phylogenetic or physiological significance of this phenomenon. But 
since it does not occur in some Angiosperms, its absence from Gnetum 
does not weaken the comparison between the two sacs which it is now 
sought to establish. 
It has been suggested above that the free nuclei fuse because they are 
free — in other words, that the physical and physiological conditions of the 
sac are favourable to the fusion of nuclei whose situation imposes no obstacle. 
This is the view suggested by Coulter * for the Angiosperms as by the writers 
for Gnetum and Welwitschia. If it is true for the Angiosperm, the sperm- 
nucleus which has escaped the influence of the egg-apparatus t may be drawn 
into fusion with the free sac-nuclei by the influences which cause these tO' 
fuse.+ In such a case the participation of the male nucleus is, in its origin, 
of no more significance than that of any of the sac-nuclei. + It may neverthe- 
less exercise an important influence in increasing the vigour of the fusion- 
nucleus as it does in introducing foreign characters into the endosperm. 
Since the endosperm can be formed without it, the view that the endosperm, 
which results from the triple fusion or its equivalent fas in Uuphorhia 
procera), is a monstrous embryo appears to be without justification. 
It seems, then, that some of the characters of the Angiosperm-sac which 
have not yet been satisfactorily explained, may be more or less accounted for 
on the hypothesis that this sac has had its origin in a less specialised form 
not necessarily very different from that now found in Gnetum. Such a 
derivation implies a very considerable shortening of that portion of the life- 
cycle which connects the macrospore-mother-cell with the functional gamete. 
It remains to inquire whether there is any evidence of a tendency to accom- 
plish such a shortening of this series of events. The Gnetales at present 
furnish no certain evidence on this point. It can therefore only be looked 
for among the Angiosperms themselves ; if it has had an effect in producing 
the Angiosperm type from a form essentially similar to that of Gnetum, it is 
not improbable that it continues to act among the Angiosperms. 
Eecent writers on the life-history of the Angiosperm have recognised the 
existence of a tendency to include in the embryo-sac § the results of the 
reduction divisions of the mother-cell (see Table II). The results of this 
tendency are seen in Columns iii-vi, ix, and to a less extent in viii and x, if, 
as is probable, these are correctly regarded as derived from a more ancient 
* Coulter, 1911. 
t In Naias major, e. g. the second sperm-nucleus may fuse either witli a synergid 
or with the polars (Coulter and Chamberlain, loc. ext.). 
X Coulter, loc. ext. 
§ Johnson, 1914, pp. 384 et seq. 
