OV THE UNIMPREGNATED OVULUM. 451 



appears to be of greater importance as connected with 

 fecundation, than as affording protection to the nucleus at 

 a more advanced period. For in many cases, before im- 

 pregnation, its perforated apex projects beyond the aper- 

 ture of the testa, and in some plants puts on the appearance 

 of an obtuse, or even dilated stigma; vrhile in the ripe 

 seed it is often either entirely obliterated, or exists only as 

 a thin film, which might readily be mistaken for the epi- 

 dermis of a third membrane then frequently observable. 



This tkird coat is formed by the proper membrane or 

 cuticle of the Nucleus, from whose substance in the unim- 

 pregnated ovulum it is never, I believe, separable, and at 

 that period is very rarely visible. In the ripe seed it is 

 distinguishable from the inner membrane only by its apex, 

 ' which is never perforated, is generally acute and more 

 deeply coloured, or even sphacelated. 



The membrane of the nucleus usually constitutes the 

 innermost coat of the seed. But in a few plants an ad- 

 ditional coat, apparently originating in the inner membrane 

 of Grew, the vesicula colliquamenti or amnios of Malpighi, 

 also exists. 



In general the Amnios, after fecundation, gradually en- 

 larges, till at length it displaces or absorbs the whole sub- 

 stance of the nucleus, containing in the ripe seed both the 

 embryo and albumen, where the latter continues to exist. 

 In such cases, however, its proper membrane is commonly tsss 

 obliterated, and its place supplied either by that of the 

 nucleus, by the inner membrane of the ovulum, or, where 

 both these are evanescent, by the testa itself. 



In other cases the albumen is formed by a deposition of 

 granular matter in the cells of the nucleus. In some of 

 these cases the membrane of the amnios seems to be per- 

 sistent, forming even in the ripe seed a proper coat for the 

 embryo, the original attachment of whose radicle to the 

 apex of this coat may also continue. This, at least, seems 

 to me the most probable explanation of the structure of true 

 Nymphseacese, namely, Nuphar, Nymphsea, Euryale, Hy- 

 dropeltis, and Cabomba, notwithstanding their very re- 



