214 ORGANOGRAPHY. BOOK I. 



embryo (Plate VI. fig. 17. c) a plumule; it is merely coty- 

 ledons. An inequality of cotyledons is the most unusual 

 circumstance with dicotyledons, and forms a distinct approach 

 to the structure of monocotyledons : it occurs in Trapa and 

 Sorocea, in which they are extremely disproportionate. In 

 Cycas they are also rather unequal; but in a much less 

 degree. 



The embryo of Monocotyledons (Plate VI. fig. I. B. &c.) 

 is usually a solid, cylindrical, undivided, homogeneous body, 

 slightly conical at each extremity, with no obvious distinction 

 of radicle, plumule, or cotyledons. In germination the upper 

 end swells and remains within the testa (fig. 10. C. Z>, &c.); 

 the lower lengthens, opens, and emits one or more radicles ; 

 and a thread-like green body is protruded from the upper 

 part of the portion wliich is lengthened beyond the testa. 

 Here the portion remaining within the testa is a single coty- 

 ledon ; that which lengthens, producing radicles from within 

 its point, is the caulicule and radicle; and the thread-like 

 protruded green body is the plumule. If this is compared with 

 the germination of dicotyledons, an obvious difference will be 

 at once perceived in the manner in which the radicles are 

 produced : in monocotyledons they are emitted from within 

 the substance of the radicular extremity, and are actually 

 sheathed at the base by the lips of the passage through which 

 they protrude; while in dicotyledons they appear at once 

 from the very surface of the radicular extremity, and con- 

 quently have no sheath at their base. Upon this difference 

 in economy, Richard proposed to substitute the term Endo- 

 rhizae for monocotyledons, and Exorhizse for dicotyledons. 

 Some consider the former less perfect than the latter : endo- 

 rhizae being involute, or imperfectly developed; exorhizae 

 evolute, or fully developed. Dumortier adds to these names 

 endophyllous and exophyllous ; because the young leaves of 

 monocotyledons are evolved fi'om within a sheath {coleophyl- 

 lum or coleoptilum), while those of dicotyledons are always 

 naked. The sheath at the base of the radicle of monocotyle- 

 dons is called the coleorhiza by Mirbel. Another form of 

 monocotyledonous embryo is that of Aroideae and their allies, 

 in which the plumule is not so intimately combined witli the 



