156 



COMPOUND ORGANS OF PLANTS. 



progresses, one of the cells towards the apex of the nucleus 

 expands, forming a cavity in its interior, termed the embryo 

 sac, because it is in this cavity, after impregnation, that the 

 rudimentary embryo first makes its appearance. 



In the mistletoe the ovule remains in this simple and naked 

 condition. Fig. 57 is the ovule of the mistletoe entire and in 

 section with the embryo sac, c. In most plants, however, the 

 nucleus becomes surrounded by one or more coverings during 

 the progress of growth. These first appear around the base of 

 the nucleus in the form of circular swellings, which gradually 

 spread over its surface, 



Fig. 57. 



In some cases, as in the ovules of the Walnut, fig. 59, 

 Fig. 58. Fig. 59. 



f 



the nucleus n, has only one covering formed on its surface ; 

 generally, however, whilst this envelope is increasing, another 

 envelope jis formed outside of it, beginning at its base, and 



