208 COMPOUND ORGANS OF PLANTS. 



CHAPTER XVI. 



THE STRUCTURE OF THE SEED. 



THE seed of Phanerogamous plants is the fecundated ovule 

 ripe and ready for germination, enclosing in its interior a plant 

 in miniature", called the plantule or embryo, which, when there 

 are the suitable conditions, is capable of reproducing the mother 

 plant, and of again passing through precisely the same phases 

 of development. 



The seed like the ovule is composed of a kernel or nucleus, " 

 usually covered by two cellular integuments, and included 

 under the general name of episperm. 



The episperm or proper tegument of the seed is the coat 

 which covers it exteriorly. This coat is formed by the two 

 membranes which we have seen to exist in the ovule at the 

 moment of fecundation, viz., the primine and secundine. In 

 a great number of cases these two membranes are so soldered 

 together that the episperm is thin and constitutes only a simple 

 membrane. But it sometimes happens that the two superposed 

 membranes of the episperm are distinct enough; and when 

 this is the case the exterior membrane is ordinarily more thick 

 and tough than the interior one, immediately enveloping the 

 seed. To distinguish them from each other, the former is 

 called the testa, and the latter the tegmen. These two mem- 

 branes are perfectly distinct in the episperm or seed coat of the 

 Castor oil plant (Ricinus.) 



The episperm has usually, on its exterior surface, certain 

 markings which correspond to those mentioned in the ovule. 



