Box.— Vol. I.] CAMPBELL— MAIAS AND ZANNICHELLIA. 



25 



At the time that the pollen-tube enters the embryo-sac, 

 the apical cells of the nucellus enlarge and their walls stain 

 more strongly than before, as if there had been a change 

 in their composition. Probably they become somewhat 

 mucilaginous and thus are more easily forced apart by the 

 growing pollen-tube. The latter, after it has entered the 

 micropyle, pushes down between the cells of the nucellus 

 (figs- 57» 58), forcing them apart and finally penetrating the 

 wall of the embryo-sac. After it has entered, the end en- 

 larges, sometimes very much (fig. 58), and this seems to be 

 at the expense of the contents of the part of the tube lying 

 outside of the embryo-sac, which in the specimens seen was 

 much collapsed. The pollen-tube was not nearly so con- 

 spicuous as Schaffner figures it for Alisma, but the further 

 history corresponds closely with his account. As in other 

 cases that have been described, one of the synergidaa ap- 

 pears to be regularly destroyed by the growth of the pollen- 

 tube close to it, and collapses completely, while the other 

 remains intact for some time longer. For demonstrating the 

 presence of the pollen-tube within the embryo-sac, the ani- 

 lin-safranin method recommended by Schaffner (1896, p. 

 123) was found to give good results, although here, too, I 

 failed to see the centrospheres, which he figures and de- 

 scribes in Alisma. One or both of the generative nuclei 

 could be seen and, in some cases at least, had changed 

 form, becoming somewhat elongated. The actual penetra- 

 tion of the generative nucleus into the Q.gg and its fusion 

 with the egg-nucleus, were not seen; but there is no reason 

 to doubt that it takes place in the usual way. 



VII. The Embryo. 



The embryogeny of N. major has been worked out by 

 Hofmeister^ but his paper was not accessible to me, so that 

 I cannot say how his results compare with my own on N. 

 flexilis, which are here given. Some important differences 



1 Hofmeister— " Neue Beitrage zur Kentitniss der Kmbryobildung der Phaneroga- 

 men." I,eipzig, 1861. 



