THE EMBKYO. 313 



of the embryo within, and of its parts, may often be inferred 

 without dissection. But the dissection of seeds is not generally 

 difficult. 



611. The direction of the radicle with respect to the pericarp 

 is also noticed by systematic writers ; who employ the terms 

 radicle superior or ascending when this points to the apex of the 

 fruit ; radicle inferior or descending when it points to its base ; 

 centripetal, when turned toward the axis of the fruit ; centrifugal 

 (or peritropous} , when turned toward the sides ; and vague, 

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

 pericarp. 



612. The position of the embryo as respects the albumen, 

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

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

 .as in all Grasses and cereal Grains (Fig. 56-61), in Polygonum, 

 &c. When external or nearl}- so, and curved circularly around 

 the albumen, as in Chickweed (Fig. 690) and Mirabilis (Fig. 

 17), it is said to be peripheric. 



613. The embryo ma}* be very variously folded or coiled in 

 the seed. The two cotyledons, instead of plane and straight, 

 may be crumpled ; or they may be simply convolute or rolled up 

 from one edge, as in Calycanthus (Fig. 691) ; or circinately con- 



volute from the apex, as in Bunias ; or else doubled up and thus 

 biplicately convolute, as in Sugar Maple, Fig. 2. Two modi- 

 fications are more common, and are of such classificatory impor- 

 tance in Cruciferae as to need special reference. Namety, when 

 cotyledons are 



Incumbent (as in Fig. 692, 693), being so folded that the back 

 of one is laid against the side of the radicle ; and 



Accumbent (Fig. 694, 695), when the edges of the pair of 

 cotyledons are longitudinally applied to the radicle. These 

 differences were first emplo3 r ed in the classification Cruciferae by 



FIG. 691. Convolute embryo of Calycanthus, the upper half cut away. 



FIG. 692. Seed of a Cruciferous plant (Sisymbrium), with incumbent cotyledons, 

 divided. 693. Embryo of the same detached entire. 



FIG. 694. Seed of a Cruciferous plant (Barbarea) with accumbent cotyledons 

 695. The embryo entire. 



