CHAPTER XVIII 

 SEED STRUCTURE 



A SEED is a fertilized ovule and consists of coats, 

 nourishing materials and an embryo. The coats are 

 usually two in number, the testa or outer and the teg- 

 men or inner coat, although a third coat or aril may 

 be developed outside the testa. The coats may be 

 merged in such a manner that their structure is dif- 

 ficult to discern. The embryo, or miniature plant, 

 contains or is surrounded by a store of nourishment 

 termed endosperm. The term perisperm is applied 

 to portions of the nucellus of the ovule, when these 

 persist in the seed. A certain amount of nourishing 

 material may be stored in the nucellus. The tissues 

 present in a seed, in the order of their position, begin- 

 ning with the outermost, are as follows: 



1 . Epidermis or testa, 



2. Inner epidermis or tegmen, 



3. Endosperm tissues, 



4. Embryo tissues. 



It is difficult to give general descriptions of the 

 cellular elements of seeds for, like fruits, they vary 

 so greatly that it is impossible to select any one seed 

 as a type. In the following notes, two types of seeds 

 will be described. In the first type, of which wheat 

 is selected as a representative, the nourishment, or 

 endosperm, is stored outside of the embryo. In the 



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