122 BOTANY 



outgrowth of the pollen grain the pollen tube pene- 

 trates the style, reaches the cavity of the ovary, makes 

 its way to the micropyle of an ovule -(Fig. 46). During 

 its progress it develops two sperms, which are quite free 

 from cilia or motile organ of any kind. The embryo sac 

 meanwhile develops its prothallus, which consists of a 

 group of cells at either end and a large nucleus in the 

 centre. One of the cells at the apical end is the ovum. 

 The fusion of the walls of the pollen tube and embryo 

 sac takes place and the two sperms enter the megaspore. 

 One fuses with the ovum, the other with the large 

 nucleus in the centre of the sac. 



These fusions constitute what is known as fertilisation. 



CHAPTER XVII 



FORMATION OF THE SEED AND ITS MIGRATION 

 THE FRUIT 



AFTER a very short interval further development begins. 

 The ovum in each case clothes itself by a cell wall and 

 certain complicated divisions take place which we cannot 

 consider in detail here. They result in all cases in the 

 formation of an embryo, or young sporophyte, which 

 remains inside the embryo sac. In the Gymnosperm it 

 is surrounded from the first by the prothallar tissue; 

 in the Angiosperm its development is accompanied by 

 the formation from the fertilised nucleus of the embryo 

 sac of a similar mass of cells, also known as the endo- 

 sperm. These changes are accompanied by considerable 

 growth of the ovule. Its integuments not only increase 

 in size but become chemically altered, generally dry and 

 hard. The embryo sac usually grows at the expense of 

 the substance of the ovule till it has absorbed the whole 

 of the latter except the integuments. In some cases, as 

 the bean with which we began our study, the young 



