FERTILISATION AND ITS EFFECTS 



275 



The development of the embryo of a dicotyledonous plant 

 from the fertilised ovum may be easily studied in the common 

 weed Shepherd's-purse {Capsella Bursa-pastoris L.). 



The ovum first surrounds itself with a cell-wall and subse- 

 quently divides into two cells : of these, the upper one or that 

 nearest the micropyle, by further transverse divisions gives rise to 

 a single row of cells termed the suspensor (s, Fig. loo). The other 



Fig. ioo. — I. Diagram of ovum. 2. The same after first division. 3 and 

 4. Suspensor (j) and embryo-cell (e) of Shepherd's-purse ; in 4 the embryo- 

 cell {/) has undergone division ; A, hypophysis. 5. Later stage of tile de- 

 velopment of the embryo, showing portion of the suspensor still attached 

 to it ; ei dermatogen ; « periblem ; p plerome of embryo. 6. Fully-formed 

 « embryo ; r its radicle ; c two cotyledons. 



or lower spherical cell (e) is carried at the end of the suspensor 

 some distance into the cavity of the embryo-sac ; it is spoken 

 of as the embryo-cell since from it the whole of the embryo is 

 developed except the tip of the radicle and the root-cap. 



The single embryo-cell divides in three directions so that eight 

 cells are formed : four of these nearest the suspensor by continued 

 division produce the hypocotyl and radicle, while the other four 

 give rise to the cotyledon and plumule of the embryo. The 



