334 THE SEED. 



632. The two lobes, or rudiments of the first pair of leaves, are 

 termed COTYLEDONS ; the bud, which, if not actually visible in the 

 seed, as in the Almond (Fig. 458, a), appears between them when 

 germination commences, is called the PLUMULE ; and the portion 

 below, which gives rise to the root, is named the RADICLE. 



633. In these illustrations, we have assumed the embryo with a 

 pair of cotyledons to be the typical, as it is the most common form, 

 occurring as it does in all the families of Exogenous plants (186). 

 Hence the latter are also called DICOTYLEDONOUS PLANTS (188). 



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

 or at least only one on the primary node ; if two rudimentary 

 leaves are present, one of them is alternate with the other, and be- 

 longs to a second node. Hence Endogens are also termed MONO- 

 COTYLEDONOUS PLANTS. The monocotyledonous embryo does 

 not usually present the same manifest distinction into radicle, 

 cotyledons, and plumule, as the dicotyledonous ; but often appears 

 like a homogeneous and undivided cylindrical or club-shaped 



body, as in Triglochin (Fig. 460). In 

 this, as in many other monocotyledo- 

 nous embryos, however, a vertical slit, 

 or chink, is observed near the radicular 

 extremity, through which the plumule 

 is protruded in germination. If the 

 embryo be divided parallel with this 

 slit, the plumule is brought into view ; 

 as in Fig. 461. If a horizontal section be made at this point (as 

 in Fig. 46*2), the cotyledon is found to be wrapped around the in- 

 closed plumule, sheathing it, much as the bud and the younger parts 

 of the stern are sheathed by the bases of the leaves in most mono- 

 cotyledonous plants. The plumule is more manifest in Grasses, 

 especially in the cereal grains, and more complex, exhibiting the 

 rudiments of several concentric leaves, or of a strong bud, previous 

 to germination (Fig. 463-465). In many cases, however, no dis- 

 tinction of parts is apparent until germination commences ; as in 

 the Onion, the Lily, &c. 



635. The more common of the extremely varied forms under 



FIG. 459. Seed of Triglochin palustre ; the raphe, leading to the strong chalaza at the sum- 

 mit, turned towards the eye. 460. The embryo detached from the seed-coats, showing the lon- 

 gitudinal chink at the base of the cotyledon ; the short part below is the radicle. 461. Same, 

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

 concealed within. 462. A cross-section through the plumule, more magnified. 



