210 THE REPRODUCTIVE ORGANS 



The differences between ovule and seed are important 

 and may be stated as follows : 



Ovule. Seed. 



Before fertilization. After fertilization. 



2 ovule coats 2 ovule-coats Seed-coat 



Nucellus Nucellus [Nucellus and 



Embryo-sac Embryo-sac endosperm used 



Endosperm up as food for] 



Egg-cell Young embryo Embryo 



Sometimes only part of the endosperm is used up ; the 

 embryo then is relatively small and more or less endosperm 

 persists round the embryo in the seed. Examples of this 

 occur in Wheat, Maize, Common Ash, and Castor Oil. Occa- 

 sionally some of the nucellus persists, when it is called 

 perisperm. 



Thus food-reserve may be stored in various regions in 

 a seed : in the cotyledons of the embryo (Bean), in the 

 endosperm of the embryo-sac (Wheat, Ash), and in the 

 perisperm or persistent nucellus (Water-lity, Pepper). 



The ovule is not the only part affected by fertilization ; 

 many surrounding parts are affected also : 



In the Pea, the carpel grows enormously and forms the 

 pod. In the Stock, the two carpels elongate greatly. In 

 the Strawberry, Rose, Apple, and Pear, the receptacle 

 becomes not only very large but fleshy. In the Winter 

 Cherry (Phy sails), the calyx becomes much inflated and 

 brightly coloured. In the Fig, Mulberry, and Pine-apple, 

 the whole inflorescence or parts of it become fleshy, fuse 

 together, and form a very complex aggregate fruit. 



The changes that take place as a result of fertilization 

 are, therefore, very great. The union of the two elements, 

 a nucleus from the pollen-grain and the nucleus of the egg- 

 cell, results in a stimulus to growth which is the starting- 

 point in the life-history of a new plant. The growth- 

 stimulus produces the changes we have outlined both 



