252 
ORGANOGRAPHY. 
BOOK I. 
mule, which in time forms the stem. The great mass of the 
kernel is supposed by Richard to be an enlarged radicle. I, 
however, see no reason for calling the two-iobed part of the 
embryo (Plate VI. fig. 17. c) a plumule, instead of cotyledons. 
An inequality of cotyledons is the most unusual circumstance 
with dicotyledons, and forms a distinct approach to the struc- 
ture 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. ch, &c.) ; 
the lower lengthens, opens at the point, 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 cotyledon; the body which lengthens, producing ra- 
dicles from within its point, is the cauliculus; and the thread- 
like protruded green part 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 
consequently have no sheath at their base. Upon this differ- 
ence Richard proposed to substitute the term Endorhizce for 
monocotyledons, and ExorhizcE for dicotyledons. Some con- 
sider the former less perfect than the latter : endorhizae being 
involute, or imperfectly developed : exorhizac evolute, or fully 
developed. Dumortier adds to these names endophyllous and 
exophyllous ; because the young leaves of monocotyledons are 
evolved from within a sheath {coleopliyllum or coleoptilum), 
while those of dicotyledons are always naked. The sheath 
at the base of the radicle of monocotyledons is called the 
coleorliiza by Mirbel. Another form of monocotyledonous 
