20 EXAMINATION OF [CHAP. 



which ovule in the buttercup is inverted (anatropous). 

 The ovule consists of a central cone, called the nucleus of 

 the ovule, around which central cone is a layer of cells 

 forming the coat of the ovule. This cellular coat grows 

 up around the nucleus, and closes over it, excepting at the 

 top, where a very minute aperture through the coat is 

 always left. This aperture is called the micropyle. Owing 

 to the ovule of the buttercup being inverted (a very fre- 

 quent condition), the micropyle is found at the base of the 

 ovule. 



By the time that the pollen-tube has reached the cavity 

 of the ovary, certain important changes have taken place 

 in the cells which form the nucleus of the ovule. One 

 cell has enlarged greatly, at the expense of its neighbours, 

 so as to occupy a considerable part of the nucleus. This 

 enlarged cell is called the embryo-sac, because within it 

 we find the embryo to originate. But this does not take 

 place until after the pollen-tube has reached the micropyle 

 of the ovule, and actually penetrated to the upper end of 

 the embryo-sac, against which it becomes closely applied. 

 Presently, after this contact of pollen-tube and embryo- 

 sac, a membranous cell-wall forms around a condensed 

 segregated portion of the protoplasmic contents of the 

 upper part of the embryo-sac, independently of the mem- 

 brane of the embryo-sac itself. This new cell by repeated 

 division ultimately developes into the embryo. At the 

 same time, in the lower portion of the embryo-sac, an 

 indefinite number of minute cells, originating in the same 

 manner, lay the foundation of the albumen, which in the 

 buttercup acquires, and permanently maintains, a rela- 

 tively large bulk as compared with the embryo, and the 

 ovule then becomes the young seed. 



5. We now understand why the sepals, petals, and 

 stamens may be spared so early. Their function is soon 

 accomplished, and their texture is too delicate to allow 

 them to persist, although in some flowers the calyx or 

 corolla, from a more succulent, or, on the other hand, 

 from a dry membranous texture, may remain a consi- 

 derable time after flowering. Such a calyx or corolla is 

 termed persistent. 



6. From what we have here described of the functions 

 of the different organs of the flower, the high importance 



