INTRODUCTION 21 



ovum forms a cell-wall and divides into two cells. 

 The upper forms a single row of cells, the suspensor. 

 The lower is the embryo-cell, which forms the 

 embryo, except the radicle formed from the hypo- 

 physis or tip of the suspensor. The embryo-cell 

 divides into eight cells, four to form the hypocotyl and 

 radicle, the other the cotyledons and plumule. The 

 synergidge and antipodal cells disappear. 



The secondary nucleus divides up, with cell-walls, 

 and forms the endosperm which serves as a reserve 

 for the embrj^o. It is used up before the mature 

 seed is formed in some plants, in others it is not. The 

 nucellar tissue is also largely used up as the embryo 

 develops, but may be reserved as perisperm. 



In the ovule the ovum becomes the embryo, the 

 integuments form the seed-coats, but the micropyle 

 and funicle remain as at first. The stigma and style 

 wither and fall. The ovule grows, as does the ovary 

 or fruit-wall. Thus fruit is formed. 



The process of fertilisation is preceded by polli- 

 nation, the course of which has already been briefly 

 outlined in the Introductory Volume. Cross-polli- 

 nation resulting in cross-fertilisation is apparently 

 the most successful in relation to the production of 

 fertile and good seed. 



The crossing of varieties is frequent in nature and 

 is largely resorted to in horticulture. Crossing 

 between different species or even genera, the latter 

 rare, is hybridity. It is rare in nature,"^ but frequently 



* But less so than has been thought. 



