14 STUDY OF COMMON PLANTS. 



that the structure of seeds is an important factor in the 

 determination of relationship. 1 



This being the case, it becomes necessary to formulate 

 certain general conceptions of form and structure, and to 

 Morphology adopt descriptive language by which they may 

 of seeds, k e expressed with clearness. 2 



The essential parts of a seed are the protective coats 

 and the embryo with its store of food. The seed-coats 

 commonly show a division into an external, 

 hard, often colored, layer, the testa, and an in- 

 ternal, more delicate one, the endopleura; the former 

 term, however, is frequently employed to designate the 

 coats collectively. In many species the endopleura is 

 wanting. Externally the testa may be smooth and pol- 

 ished, as is the case with the seed of the castor oil plant, 

 or it may be covered with hairs, as cotton seeds are, or, 

 again, it may be extended into a wing, like that belong- 

 ing to the seeds of the catalpa, and various other modifi- 

 cations may occur, having, as a rule, a direct relation to 

 protection or dissemination. An additional coat, usually 

 colored and fleshy, known as the aril, is rarely present. 



The parts of the embryo are the radicle, cotyledons, and 

 plumule. As we have seen, it may have one, two, or sev- 

 eral cotyledons, and accordingly is said to be 

 monocotyledonous, dicotyledonous, or polycoty- 

 ledonous. The embryo varies greatly in different species 

 as regards form, position, and size, being straight or 

 curved ; occupying the whole space within the seed-coats, 

 or only a small portion of it ; the cotyledons alike or dif- 



1 See, for example, Rowlee, Bulletin of the Torrey Botanical Club, 

 XX (1893), p. 1, and Rolfs, Botanical Gazette, XVII (1892), p. 33. 



2 For a more extended treatment of the morphology of seeds cf . Gray, 

 Structural Botany. 



