THE EMBRYO. 



335 



which the embryo occurs may readily be gathered from the nu- 

 merous illustrations scattered through this volume ; which need not 

 be specially enumerated. Its position as respects the albumen, 

 when that is present, is also various. Although more commonly 

 in the axis, it is often excentric, or even external to the albumen, 

 as in all Grasses (Fig. 463-465), in Polygonum (Fig. 787), &c. 



When external or nearly so, and curved circularly around the 

 albumen, as in Fig. 559, 565, 995, and generally in the fami- 

 lies from which these illustrations are taken, it is called peripheric. 

 When the embryo is bent so that the radicle is placed against the 

 edges of the cotyledons, the latter are said to be decumbent ( Fig. 

 529) ; or when the radicle rests against the back of one of them 

 (Fig. 538), they are called incumbent. 



636. The situation of the embryo with respect to the base and 

 apex of the seed is so far uniform, that the radicle always points to 

 the micropyle, as already mentioned. As the nature of the seed 

 may usually, after some practice, be readily determined by exter- 

 nal inspection, so the situation of the embryo within, consequently, 

 may often be inferred without actual dissection. 



637. The direction of the embryo with respect to the pericarp is 

 also particularly noticed by systematic writers ; who employ the 

 terms ascending, or radicle superior, when the latter points to the 

 apex of the fruit; descending, or radicle inferior, when it points 

 to its base; centripetal, when the radicle is turned towards the 

 axis of the fruit ; centrifugal, if towards the sides ; and vague, 

 when it bears no evident or uniform relation of this kind to the 

 pericarp. 



638. Sometimes the two cotyledons of a dicotyledonous embryo 



FIG. 463. Vertical section of a grain of Indian Corn, passing through the embryo : c, the 

 cotyledon ; p, the plumule ; r, the radicle. (A highly magnified portion of the albumen, which 

 makes up the principal bulk of the grain, is shown in Fig. 52, p. 57.) 464. Similar section of 

 a grain of Rice. 465. Vertical section of an Oat-grain: a, the albumen; c, the cotyledon ; p, 

 the plumule ; and r, the radicle of the embryo. 



