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. J, Sec); 
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 which 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- 
rhizse 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 
endophylhus and exophyllous ; because the young leaves of 
monocotyledons are evolved from 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 with the 
