MONOCOTFLEDONS. 54 j 



CLASS XII. 



MONOCOTYLEDONS. 



The Setd of Monocotyledons usually contains a strongly developed endosperm 

 and a comparatively small embryo; and this is exhibited in an especially striking 

 manner in large seeds, such as those of Cocos, Phoenix, Phytelephas, Crinum, &c. 

 In the Naiadeae, Juncagineee, Alismaceae, and Orchidese, the endosperm is wanting 

 from the first ; and in the Scitaminea^, where it is also wanting, it is replaced by a 

 copious perisperm. 



The Embryo is usually cylindrical, fusiform, and sometimes considerably 

 elongated, and is then also curved spirally {e.g. in Potamogeton and Zanichellia) ; 

 its form is not unfrequently that of an erect or inverted cone, in consequence of 

 a considerable thickening of the upper end of the cotyledon. The axis of the 

 embryo is generally very short and small in comparison to the cotyledon; in 

 the Helobiae on the contrary the axial portion of the embryo forms the greater 

 part of it. At the posterior end of the axis is the rudiment of the primary root, 

 in addition to which two or more lateral roots also originate in Grasses, which, like 

 the primary one, are surrounded by a root-sheath (Fig. 114, p. 144). The embryo 

 of Grasses is also distinguished by the presence of the Scutellum, an outgrowth 

 of the axis beneath the cotyledon, which envelopes the whole of the embryo 

 like a mantle, and forms a thick peltate plate on the posterior side where it is in 

 contact with the endosperm \ In the Orchidece, Apostasiaceae, and Burmanniaceae, 

 the parts of the embryo of the ripe seed are not differentiated; it consists of a 

 round mass of tissue on which the plumule is developed only during germination. 



Germinatioti^ either begins at once with the lengthening of the roots — their 

 protrusion causing in Grasses the rupture of the root-sheath which envelopes them, 

 and which remains attached to the axis of the embryo as the Coleorhiza (Fig. 113, 

 p. 143) — or, as is more commonly the case, the lower part of the cotyledon 

 lengthens, and pushes the end of the root, together with the plumule which is 

 enveloped by the sheath of the cotyledon, out of the seed (Fig. 388), while its 

 upper part remains in the endosperm as an organ of absorption, until the 

 endosperm is consumed. In Grasses, however, the whole of the plumule projects 

 from the seed, the scutellum only remaining behind in it, in order to convey to 

 the embryo the food-material contained in the endosperm. 



' [Van Tieghem (Ann. des Sci. Nat. 5th series, vol. XV, 1R72) gives a useful summary of the 

 various views which have been held with respect to the homology of the parts of the embryo of 

 Grasses. He regards the scutellum as the cotyledon, and what Sachs considers the cotyledon as 

 only its strongly developed ligule. — Ed.] 



2 See Sachs, Bot. Zeit. 1862 and 1863. 



