346 



ORGANOGRArHY. 



Fig. 743. Progressive devel- 

 opment of a dicotyledonous 

 embrvo. 1. Earliest stage. 

 2, 3. Stages of progression. 

 4. Most developed. 



Fig. 743. ^ the development of the embryo is com- 



pleted. The different stages in the 

 development of the embryo are well 

 illustrated in figure 743, 1, 2, 3, 4. 

 From the axil of the cotyledons, the 

 plumule is subsequently developed. 

 The foraiation of the monocotyledonous 

 embryo is essentially the same, except 

 that the lower end remains undivided. 

 From this mode of development of the 

 parts of the embryo, it must necessarily 

 follow, that the radicle is pointed to- 

 wards the apex of the nucleus or 

 micropyle {fig. 734, w), and the cotyledonary portion towards 

 the opposite extremity or chalaza. 



There are some natural orders which offer an exception to the 

 above process of development. Thus in the Orchidaceai, Oro- 

 banchaceae, and Balanophoracea, the radicle and cotyledons are 

 never clearly distinct from each other, but the embryo appears 

 to be arrested at one of the early stages of its development. 



It sometimes happens that more than one embryo is developed 

 Fig. 74i. Fig. 7 A5. in a seed. This is very commonly the case 

 in the Orange, Mistletoe, and especially in 

 Gymnospennous Plants ; of these embryos, 

 only one usually becomes perfectly developed. 

 Plants thus producing more than one embryo 

 are said to be poly embryonic. With these 

 general remarks upon the development of tlie 

 embryo, we now proceed to the description 

 of that of Monocotyledonous and Dicotyle- 

 donous Plants. 



a. The Monocotyledonous Embryo. — The 

 parts of the monocotyledonous embryo are, 

 in general, by no means so apparent as those of 

 the dicotyledonous. Thus the embryo at first 

 sight externally, usually appears to be a solid 

 undivided body of a cylindrical or somewhat 

 club-shaped form, as in Triglochin {fig. 745); 

 if this be more carefully examined, however, 

 a little slit,/, or chink, will be observed on 

 one side near the base; and if a vertical sec- 

 tion be made parallel to this slit, a small 

 conical projection will be noticed, which cor- 

 the* ''emmuic"^r"co° ^'^sponds to tlic plumulc ; and now, by making 

 tyiedon. From Jus- a horizontal section, the single cotyledon will 

 "«"• be noticed to be folded round the i)lumulc, 



which it had thus almost entirely removed from view, only 

 leaving a little slit corresporiding to the uriiou of the margins 



Fig. 744. Vertical sec- 

 tion of a carpel of a 

 Bpecies of Tri(jlo- 

 chin. p. Pericarp. 

 «. Stigma, y. Seed. 

 r. Raphe. /. Fu- 

 niculus, r. Cha- 



Jaza Fig. 745. 



Embryo of Triglo- 

 chin. r. Radicle. /. 



