THE EMBRYO-SAC OR FEMALE GAMETOPHYTE. 



171 



rise to the axial row of four cells, correspond, as in other Gymno- 

 sperms, precisely with the first and second divisions in the microspore 

 mother-cell of this plant. In my own opinion the only legitimate 

 conclusion to be drawn from this morphological and cytological evi- 

 dence is that the macrospore mother-cell of Larix is homologous with 

 that of Helleborus and other Angiosperms in which the embryo-sac 

 develops similarly. 



In the development of the embryo-sac, as typified by Lilitim and 

 many other monocotyledonous plants, the hypodermal cell does not 

 produce an axial row of four cells, but becomes at once the functional 

 macrospore. With the growth of the nucellus this hypodermal cell 

 increases greatly in size, as does also its nucleus (Fig. 71). The 

 nucleus, after its characteristic period of 

 growth, divides heterotypically. The two 

 resulting daughter-nuclei lie in the ends of 

 the cell. No cell-division follows this 

 nuclear division, although the thickening of 

 the connecting fibers in the equatorial region 

 seems to indicate that a tendency toward cell- 

 division existed (Fig. 72, A). The macro- 

 spore continues its growth, and the daughter- 

 nuclei divide. This division is homotypic 

 and corresponds exactly to the second mitosis 

 in the pollen mother-cell. The four resulting 

 nuclei have, as a rule, the orientation shown 

 in Fig. 72, B. Very frequently no vacuole 

 is present at this stage, and the four nuclei are 

 connected with each other and with the plasma 

 membrane by systems of kinoplasmic radiations and connecting fibers. 

 The increase of the cell in length is now rapid, and, as a result, one 

 or more large vacuoles are formed at the center or near the micropylar 

 end of the sac. Two of the four nuclei which are sisters move into 

 the upper, and the other two into the lower end of the cell. In normal 

 cases the nuclei in each end divide so that a group of four nuclei occu- 

 pies each end. The four nuclei in the micropylar end are arranged 

 either in a plane, or nearly so, or in the form of a tetrad (Fig. 73, 

 A, B). The arrangement and behavior of the nuclei in the chalazal 

 end of the sac is more variable (Mottier, '97). 



As a rule the two nuclei in the micropylar end of the sac, and it is 

 with these that we are especially concerned, divide simultaneously, and, 

 before cell-plates are laid down, the four resulting nuclei are connected 



FIG. 71 Embryo-sac mother-cell 

 of Liliunt martagon with nu- 

 cleus showing beginning of 

 prophase of division. 



