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 Coniferee they vary 

 from two to more than twelve. 



Instances of the absence of cot}4edons 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 Lentibularise ; 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 



