GROUP IV. PHANEROGAMIA : ANGIOSPERMJi ; MONOCOTYLEDONES. 533 



The Angiosperms are subdivided as follows : 



Class IX. MONOCOTYLEDONS : the embryo has usually a single 

 terminal cotyledon, and the growing-point of the primary stem is 

 developed laterally : the ripe seed usually contains abundant endo- 

 sperm : the vascular bundles of the stem are closed : the leaves 

 commonly have parallel venation: the flower belongs usually to 

 the pentacyclic trimerous type. 



Class X. DICOTYLEDONES : the embryo has usually two opposite 

 cotyledons, and the growing-point of the primary stem is de- 

 veloped terminally : the ripe seed is commonly exalbuminous : 

 the vascular bundles of the stem are usually open : the leaves 

 commonly have reticulate venation : the structure of the flower 

 varies, but it frequently belongs to the pentacyclic pentamerous 

 type. 



CLASS IX. MONOCOTYLEDONES. 



Although the seed typically contains endosperm, it contains 

 none in certain orders; namely, the Orchidaceae, most aquatic 

 Monocotyledons (Alismales, Hydrocharidacese), and in some genera 

 of AraceaB (Orontium, Symplocarpus, Scindapsus, Monstera, 

 Amorphophallus). In the Scitamineaa perisperm is always 

 present in the seed, either together with endosperm (Zingi- 

 beracese), or without endosperm (Musacese, Marantacese). In the 

 albuminous seeds, the embryo is usually small in proportion to 

 the endosperm (Fig. 345 I, e, c). 



Whilst the single cotyledon of the embryo is, as a rule, terminal 

 and the growing-point of the stem lateral, in some forms the 

 growing-point of the stem is terminal (apical) on the longitudinal 

 axis of the embryo (Dioscoreaceae, Commelynacese, see p. 445). 

 The growing-point of the primary stem frequently developes into 

 a plumule. The axis of the embryo terminates posteriorly in a 

 short radicle. 



On germination, the upper end of the cotyledon commonly 

 remains in the seed and absorbs the nutritious substances de- 

 posited in the endosperm (Fig. 345 II -IV.) ; the lower part of the 

 cotyledon elongates and pushes the rest of the embryo out of the 

 seed. In Grasses the cotyledon has a peculiar shield-like form, and 

 is termed the scutellum (Fig. 346 sc) : in the ripe seed it almost 

 entirely encloses the embryo, and is in contact by its outer surface 



