152 ELEMENTS OF STRUCTURAL BOTANY. 



the embryo-sac which is next the micropyle, and about 

 the same time, in the other end of the embryo-sac, tnere 

 begins a deposit of matter intended for the nourishment 

 of the embryo during the germination of the seed. This 

 deposit has been already referred to under the name of 

 albumen. It is also known as endosperm. As the embryo 

 developes, this endosperm or albumen may be completely 

 absorbed by it, so that at maturity the embryo will occupy 

 the whole space within the seed-coats, as in the Bean. In 

 this case the seed is exalbuminous. In other cases, as in 

 Indian Corn, the endosperm remains as a distinct mass 

 with the embryo embedded in it, or sometimes wrapped 

 round it. Seeds of this kind are albuminous. Rarely 

 this nourishing material is deposited outside the embryo- 

 sac, in the body of the ovule. It is then known as 



249. The ripened seed presents very different aspects 

 in different plants. It may be resolved, however, into the 

 nucleus and the integument (the spermoderm of some 

 botanists). The former is made up of the embryo, 

 together with the endosperm or perisperm, if present, 

 while the latter consists of two layers : an outer, known 

 as the testa, and an inner, the tegmen. The scar showing 

 where the seed has been attached to the placenta is called 

 the hilum ; it is very distinct in the Bean. 



250. Besides the integument just mentioned, occasionally 

 an additional outer coat is formed, to which the term aril 

 is applied. The fleshy red covering of the seed in our 

 Ground Hemlock is a good example. 



'251. The seeds of Willow-herb, Milkweed, and many 

 otner plants are furnished with tufts of hair-like bristles 



