DICOTYLEDONOUS EMBRYO. 



285 



an enlarged mammillary projection, whence the rootlets proceed by 

 bursting through it, and carrying with them a covering or sheath 

 (f 127, fig. 124.) 



596. When considering endogenous or 

 monocotyledonous stems, it was shown that 

 the leaves are produced singly and alter- 

 nately, in a sheathing manner, each em- 

 bracing the subsequently developed bud. 

 So it is in the monocotyledonous embryo. 

 There is a single leaf or cotyledon pro- 

 duced, and if in any instance there is more 

 than one, it is alternate with the first 

 formed. The cotyledon (fig. 504 c) is 

 folded either partially, as in Dioscorea, or 

 completely. Its sheathing portion (vagina) 

 embraces the bud or gemmule, which - 

 appears as a mammillary projection ; its 

 position being indicated by a cleft or slit 

 (fig. 504/), where the edges of the sheath 

 unite. All the portion of the embryo above 

 the gemmule, is the cotyledon ; all below, 

 the radicle. 



597. Dicotyledonous Embryo. The form 



of this embryo varies much ; and although 

 sometimes resembling in its general aspect 

 that of monocotyledons, yet it is always 

 distinguished by a division taking place at 

 the cotyledonary extremity, by which it is 

 separated into two, more or less evident, 

 lobes. The parts of this embryo are easily 

 traced in the Bean, Pea, Acorn, and Almond. 

 In the latter (fig. 500), the embryo has an 

 oval form, consisting of two thick cotyle- 

 dons, c c, and a radicle, r. When one of 

 the cotyledons is removed (fig. 501), leaving . 

 scars, i c, the gemmule or plumule, g, is 

 seen included between them, with its cauli- 

 cule or tigelle, t. 



598. The cotyledons, are not always, 



however, of the same size. Thus, in a 505 



species of Hirsea (fig. 506), one of them, c', is smaller than the other; 



and in Carapa guianensis (fig. 507), there appears to be only one, 



Fig. 505. Monocotyledonous embryo of Zannichellia palustris germinating, m, Collum or 

 neck, the point intermediate between the stem or tigelle, t, and the radicle or root, r. c, Cotyle- 

 don, g, Gemmule or plumule. 



