CHAP. II. 
EMBRYO. 
213 
the embryo consists of a homogeneous undivided mass, with 
a curved horn-like prolongation of one side directed towards 
the hilum. If a section be made in the direction of the axis 
of the horn-like prolongation through the whole mass of the 
embryo, a slit will be observable above the middle of the 
horn, at the base of which lies a little conical body. In this 
embryo the slit indicates the division between the two bases of 
a pair of opposite confluent cotyledons ; the conical body is 
the plumule, and the horn-like prolongation is the radicle. 
In Castanea nearly the same structure exists, except that 
the radicle, instead of being curved and exserted, is straight, 
and enclosed within the projecting base of the two cotyle- 
dons ; and in Tropaeolum, which is very similar to Castanea 
in structure, the bases of the cotyledons, are slit into four 
little teeth enclosing the radicle. The germination of these 
seeds indicates more clearly that the cotyledonary body con- 
sists of two and not of one cotyledon ; at that time the bases 
of the cotyledons, which had been previously scarcely visible, 
separate and lengthen, so as to extricate the radicle and 
plumule from the testa, within which they had been confined. 
In number the cotyledons vary from two to a much more con- 
siderable number. Ceratophyllum has constantly four, of 
which two are smaller than the others ; in Coniferae they vary 
from two to more than twelve. 
Instances of the absence of cotyledons occur, 1. In Cus- 
cuta (Plate VI. fig. 19.), to which they may be supposed to 
be denied in consequence of the absence of leaves in that 
genus ; 2. in Lentibulariae ; 3. in Cyclamen, in which the 
radicle enlarges exceedingly : to these a fourth instance has 
by some been added in Lecythis, of which Richard gives the 
following account. The kernel is a fleshy almond-like body, 
so solid and homogeneous that it is extremely difficult to dis- 
cover its two extremities until germination takes place : at 
that period one of the ends forms a little protuberance, which 
subsequently bursts through the integuments of the seed, and 
extends itself as a root ; the other end produces a scaly plu- 
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-lobed part of the 
p 3 
