364 THE FRUIT 



sperm, and this is supplied from the bundle which 

 terminates at the base of the nucellus. 



When the embryo is ripe it may fill the whole of the 

 space within the embryo sac, or it may still be sur- 

 rounded by a mass of endosperm (Fig. 62, E) (see p. 362). 

 The nucellus of the ovule is sometimes represented in 

 the seed by a thin layer of tissue which may be stored 

 with food substance (perisperm). The coat (or coats) 

 of the ovule becomes differentiated in various ways, 

 their cell walls generally thickened and cutinised or 

 lignified, to form the seed coat or testa. 



It will be useful here to summarise the corresponding 

 structures in ovule and seed : 



The ovule becomes the seed. 



The coat (or coats) of the ovule becomes the testa. 



The nucellus may become the perisperm. 



The tissue formed by the division of the secondary 

 nucleus of the embryo sac forms the endosperm. 



The zygote becomes the embryo (together with the 

 suspensor a transitory structure). 



Development of Fruit from Carpels, etc. While the 

 changes described above are taking place in the ovule, 

 others are proceeding in the rest of the flower. The petals 

 and stamens usually fall off very soon after fertilisation, 

 while the calyx is often, though not always, persistent, 

 and sometimes grows considerably in size. The stigma 

 and style fall off or wither away, but the walls of the 

 ovary typically develop into the walls of the fruit 

 (pericarp), keeping pace with, or even outstripping, the 

 growth of the seeds. The pericarp, into which the 

 ovary wall develops, differs very much in different 

 species, being sometimes thin and membranous (pea), 

 sometimes thick and woody (hazel nut), sometimes 

 soft and succulent (raspberry, tomato). 



