326 



THE SEED. 



way that the radicle lies along the edges of the cotyledons, the 

 latter are said to be accumbent (Fig. 700) ; or when the radicle 

 rests against the back of one of them, or in proximity to it (Fig. 

 705), they are incumbent. 



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

 also particularly noticed by systematic writers ; wlio employ the 

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

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

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

 of the fruit ; centrifugcd, when turned towards the sides ; and vague, 

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

 pericarp. 



641. As to the number of its cotyledons, or the degree of com- 

 plexity or simplicity of the embryo, the principal types have already 

 been considered (128). The plan of the embryo in Exogenous 

 plants is to have a pair of opposite cotyledons ; tliat is, the embryo 

 is dicotyledonous, and such plants are denominated Dicotyledo- 

 nous Plants. 



642. A modification of this plan occurs in Pines and most other 

 Coniferas, in which the cotyledons are increased to three, four, six, 

 or even to fifteen, in a whorl (Fig. 133, 134) ; and this embryo of 

 highest complexity is called polycotyledonous. The embryos of 

 some Leguminous or Cruciferous plants are occasionally found, with 

 three cotyledons, as an accidental deviation. 



643. But in all Endogenous plants only one cotyledon appears, 

 i. e. only one seed-leaf on the primary node ; if two or more rudi- 

 mentary leaves are present, they are alternate, and all but the first 



belong to the plumule. Here the em- 

 bryo is monocotyledonoui, and hence 

 Endogens are also tenned Mono cot y- 

 LEDONOus Plants. The monocoty- 

 ledonous embryo does not usually pre- 

 sent a manifest distinction into radicle, 

 cotyledons, and plumule, as the dicoty- 

 ledonous ; but often appears like a ho- 

 mogeneous and undivided cylindrical or club-shaped body, as in Iris 



FIU. 625. Seed of Triglochin palustre ; the rhaphe, leading to the strong chalaza at the 

 summit, turned towards the eye. 626. The embryo detached from the seed-coats, showing 

 the longitudinal chink at the base of the cotyledon ; the short part below is the radicle.' 627. 

 Same, with the chink turned laterally, and half the cotyledon cut away, bringing to view the 

 plumule concealed within. 628 A cross-section through the i/lumule, more magnified. 



