5^2 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 synergidae. 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. — 



