582 
PHANEROGAMS. 
reasoning Warming and Vesque consider that the processes of cell-formation which 
go on in the embryo-sac correspond to the formation of spores or pollen-grains 
from their mother-cell, the four nuclei at each end of the embryo-sac representing 
a tetrad of spores. In order that this view may be perfectly consistent it is obviously 
necessary to assume that the embryo-sac is formed by the fusion of two cells equiva- 
lent to spore-mother-cells, inasmuch as two tetrads of nuclei are formed within it, 
and further, that the cells which are formed round these nuclei are each of them 
equivalent to a spore. 
The view which is more generally held, and which is due to Strasburger, is that 
the cell which forms the embryo-sac is a spore-mother-cell which does not undergo 
any division into special spore-mother-cells, but which developes without dividing 
into a single spore, the embryo-sac. The processes of cell-formation which go on 
in the embryo-sac are not therefore to be compared to those which accompany the 
formation of spores or of pollen-grains from their mother-cell, but they are to be 
compared to those which accompany the germination of a spore or of a pollen- 
grain. The six cells which are formed in the embryo-sac, three at the chalazal and 
three at the micropylar end, represent a rudimentary prothallium; they are com- 
parable to the endosperm of Gymnosperms and to the prothallium in the macrospore 
of the heterosporous Vascular Cryptogams ^ These cells, which we may term 
primary endosperm-cells, are, as we have seen, subsequently differentiated into the 
antipodal cells and the egg-apparatus; thus three of them are purely vegetative, 
whilst the other three are concerned in the sexual reproduction of the plant, one 
of them becoming the oosphere and the other two the synergidse. In the Vascular 
Cryptogams we saw that the archegonium was developed from a single superficial 
cell of the prothallium ; in the Gymnosperms we saw that the archegonium was 
developed from a single superficial cell of the endosperm, that it was a less com- 
plex organ than in the Vascular Cryptogams, and that it had undergone a reduction, 
a condition which we found most evident in Welwitschia in which the mature arche- 
gonium consists of only a single cell ; in the Angiosperms this reduction is carried 
still further, the archegonium being represented only by the oosphere, which is one 
of the primary endosperm-cells. It was thought at one time that the Filiform 
Apparatus of Schacht represented the canal-cell of the archegonium which we have 
found to be present in the higher Cryptogams and in most Gymnosperms, but this 
cannot be the case inasmuch as this apparatus is, as stated above, simply the striated 
ends of the synergidae ; still less can the synergidae be canal-cells, for they are the 
product of a nuclear division and cell-formation in which the oosphere is not 
directly concerned; moreover their function is entirely peculiar. The completion 
of the prothallium takes place when what we may term the secondary endosperm 
is formed ; this is what is commonly termed the endosperm, and its formation in 
the embryo-sac does not commence until the fertilisation of the oosphere has been 
effected.] 
Fertilisation'^. The pollen-grains which germinate on the stigma send out 
^ [Allusion has already been made (on p. 486) to Goebel's view that the antipodal cells of 
Angiosperms correspond to the * endosperm ' of Selaginella,'] 
^ Besides the works of Hofmeister already quoted, see his historical account in Flora, 1857, 
p. 125, where the literature is collected. [Also Strasburger, Ueb. Befruchtung und Zelltheilung.— 
