EMBRYO. 



[ 225 ] 



EMBRYO. 



EMBRYO, OF Plants.— This is the 

 name apphed to the rudimentary plant con- 

 tained in all true seeds, which are especially 

 distinguished from spores by this character; 

 since in all plants reproduced by spores, the 

 rudiment of the new individual is developed 

 after the separation of the reproductive 

 body from the parent plant. Seeds con- 

 taining embryos are borne exclusively by 

 Flowering plants ; and while the external con- 

 ditions under which the seeds are jiroduced 

 afford the character for the first subdivision 

 of this province of the Vegetable Kingdom 

 (Angiosperms and Gymnosperms), the 

 structure of the embryo is taken as the most 

 striking character in further subdividing the 

 Angiospermous Flowering Plants into their 

 two great natural groups, viz. Monocotyledons 

 and Dicotyledons, in which, respectively, the 

 embryo bears one or two cotyledons or seed- 

 leaves. In a perfectly developed embryo we 

 distinguish three parts, the cotyledons, the 

 plumuleor bud, and iheradicleov rudimentary 

 root : the part at the base of the plumule fi'om 

 whence the cotyledons and radicle start in 

 opposite directions is sometimes called the 

 cauliculus ; in a Dicotyledonous embryo 

 there exist a pair of opposite cotyledons 

 enclosing the plumule between them, the 

 whole being seated upon the upper end of 

 the radicle : in the Monocotvledons a single 

 cotyledon is rolled round the plumule (like 

 the leaf-sheath of a grass round the stem), 

 and continuous below with the radicle, which 

 is usually less distinct than in the Dicotyle- 

 dons. Cases occur both among the Dicoty- 

 ledons and the Monocotyledons where the 

 t5q3ical structure is departed from. Thus in 

 Orobanchacese (Dicotyledons) the embryo is 

 a mere globular mass of cellular tissue, the 

 result of an arrest of development, the coty- 

 ledons and radicle never becoming distinct ; 

 the same is the case in the Orchidacese 

 among the Monocotyledons, the embryo not 

 advancing beyond the state of a globular 



A young Dicotyledonous embryo in successive stages of 

 development. All exhibit the suspensor, and 4 has the 

 cotyledons appearing, separated by a notch. Magnified 

 50 diameters. 



mass of parenchyma. The relation of such 



embryos to the perfect forms is well illustrated 

 by comparing the stages of growth of 

 embryos which acquire fully-developed coty- 

 ledons and radicle (fig. 195). The apparent 

 anomaly of such cases occurring in the 

 natural classes characterized by the presence 

 of one or two cotyledons, is of a kind familiar 

 to all botanists and zoologists ; no one cha- 

 racter of a natural group can be regarded as 

 universal and arbitrary ; it is the presence of 

 a majority that decides, and thus the above 

 plants fall under those classes, from the cha- 

 racters of their wood, fohage, flowers, &c., 

 although their embryos are devoid of coty- 

 ledons. In Cuscuta, a leafless plant, the 

 embryo has no distinct cotyledons. Other 

 anomalies of another kind also occur. Some 

 Monocotyledons, such as those of Grasses, 

 have the rudiment of a second cotyledon, 

 but this is above and not opposite the other 

 larger one. In Dicotyledons the cotyledons 

 are not unfrequently unequal, and sometimes 

 soldered together. In the Coniferse the 

 embryos appear to have four, eight, or more 

 cot3dedons in different cases, but it is stated 

 that there exist only two, divided or com- 

 pound cotyledons (see Seeds). 



Occasionally more than one embryo occurs 

 in a seed (see Polyembryony), and in the 

 Coniferse a number of embryos are at first 

 produced, of which one only becomes per- 

 fectly developed (see Gymnospermia). 



The embryo sometimes constitutes the 

 whole mass of the seed, merely enclosed in 

 the coats ; in other cases it is imbedded in a 

 mass of albumen. In the former case the 

 tissue of the cotyledons often assumes cha- 

 racters similar to those of the albumen, 

 serving as a receptacle for stored nutriment 

 for the germinating plant, in the form of 

 fleshy secondary deposits, starch, oil, &c. 

 The position of the embryo in the albumen, 

 or the modes in which the embryo is folded 

 up within the seed-coats, are of great im- 

 portance in systematic botany, for the charac- 

 terization of families. Particulars regarding 

 these points, and the manner of examining 

 them, are given under the head of Seed. 

 The development of embryos is described un- 

 der Ovule. The germination of embryos 

 is placed under its separate head. See also Or- 

 chidace^, Orobanchace^, Cuscuta. 



BiBL. General works on Structural 

 Botany ; Brongniart, Ann. des Sc. nat. xii. 

 p. 14, &c. ; Jussieu, Ann. des Sc. nat. 2 ser. 

 xi. p. 341; St. Hilaire, Lemons de Botanique, 

 Ann. des Sc.nat. 2 ser. v. p. 193; Duchartre, 

 Ann. des Sc. nat. 3 ser. x. p. 20/, and the 



Q 



