128 THE SEED 



the embryo, between the time of germination and that of absorption by 

 it from the external world, calls for additional nourishment. This can 

 be met only by the storage as a part of the seed of an additional food- 

 supply. 



Protection of the seed-contents during its development is only partially 

 afforded by the pericarp, and this office is supplemented by the coverings 

 of the seed itself, while its similar self-protection between the periods of 

 maturity and germination is a manifest necessity. 



The transfer of the mature seed to the point of germination, or its 

 dissemination, and its fixation in a favorable site, have already been 

 referred to. We have seen that in many cases these offices are not 

 provided for by the pericarp, and we must look for such provision to 

 the seed itself. 



Parts of a Seed. — The parts of the seed by which these several offices 

 are performed we find to be as follows: 



The Perisperm.- — The source of food-supply during the germination 

 of the macrospore and development of the gametophyte we have seen 

 to be the portion of the nucellus external to the embryo-sac. Usually 

 more or less of this material remains during at least the earlier period of 

 the development of the embryo and contributes to the nourishment 

 of the latter. Occasionally it persists even in the seed condition. It is 

 then known as the Perisperm. 



The Endosperm. — Inside the embryo-sac a further store of nutriment 

 is caused to develop as a result of fertilization, this constituting the 

 chief supply of the growing embryo. More or less of this also may 

 persist, and usually does, upon the maturity of the seed. It is known as 

 the Endosperm. 



Albuminous and Exalbuminous Seeds. — As the embryo develops, it 

 stores within its own body more or less nutriment. At maturity we 

 may find that the entire store of nutriment has thus been transferred 

 to the body of the embryo, and the seed is said to be Exalbuminous, or 

 we may find more or less endosperm or perisperm, or both, when the 

 seed is said to be Albuminous, and this external nourishment is known 

 as the Albumin. In only a few seeds used in medicine does the albumin 

 consist in any part of perisperm. The chemical nature of the albumin 

 is extremely variable. It received its misleading name because of the 

 similarity of its function to that of the albumin of the egg. 



Protection. — Protection to the embryo may be afforded by the albumin 

 when that is of the required consistency or composition, the conditions 

 of the latter being a mere parallel of those already considered under 



