136 COTYLEDONCUS PLANTS. 



The seed of a Dicotyledon, as the Pea or Apple, presents the following parts (Fig. 

 265) : two Cotyledons, or seed-leaves, a, at the upper part, within the base of which ia 

 a minute point, destined to become the stem, named the plumule, b; 

 and at the bottom of the seed is the radicle, c, having dimensions 

 larger than those of the plumule, and separated from the Cotyle- 

 dons by an unseen line, the caulicule. Sometimes the Cotyledons 

 are absent, or cohere into one mass, or divide into a greater num- 

 ber, as four in the Cruciferse, and double that number in some of 

 the Coniferae. 



An approach to the condition of a Monocotyledenous seed, is 

 seen in such Dicotyledons as have great inequality in the size of 

 the Cotyledons ; so that one of them is scarcely perceptible. In 

 this class (Monocotyledons, represented in this country by the . ^~^ 



Grasses), there is no such distinction of parts as that now referred the Garden Bean, 

 to ; but the lower part of the seed emits a number of radicles ^[k^radicle lu " 

 (Fig. 264 r), whilst from the upper part the thread-like green 

 plumule, p, is emitted. Thus the growing points are sheathed by the embryo, which 

 remains within the testa throughout the process of germination. There are many 

 exceptions to this description; but they do not materially invalidate the rule now 

 given. Monocotyledonous plants are as exclusively endogenous (p. 86) as dicotyledo- 

 nous are exogenous in their general structure. 



There are certain plants in which distinct Cotyledons have not been discovered, and 

 hence have been termed Acotyledons ; but this would not be a correct mark of distinction 

 for the members of the two classes now described, since it is probable that there 

 are parts analogous to Cotyledons. The true diagnostic is, that in these plants the 

 germination does not proceed from fixed points, as the plumule and radicle, but 

 indifferently from any part of the surface of the seed. This is the condition of the 

 embryo in the great class of plants to which we shall presently refer viz., the flower- 

 less plants. 



There are yet one or two points to which reference must be made, before we conclude 

 this account of the seed. The Amnios (Fig. 250) is a fleshy bag surrounding the embryo 

 in many seeds, and consequently lying within the innermost integument. It has also 

 been termed the Vitellus, or Yolk-bag, and it probably performs an analogous office in 

 the sustentation of the embryo. 



"We have already referred to the hiluni and other vascular parts of the ovule and 

 seed, and need here only to state that the hilum is the umbilicus, or the spot at which 

 the vessels from the placenta enter the seed. In many plants it can scarcely be seen, 

 whilst in others it is of a dark colour, or is very large, as the Pea, Bean, and Horse- 

 chestnut. The micropyle, or foramen, is the opening in the seed to which the radhle 

 is always directed, and may be at the end of the seed opposite to the hilum, or the two 

 may be close together, as in the Pea ; or it may occupy other positions, as shown at page 

 128, in reference to the ovule. Its position determines that of the radicle, and conse- 

 quently is of importance. The chalaza and raphe have precisely the same indications 

 in the seed as in th" ovule ; but the latter is always distinctive of the face of the seed 

 when the figure of that organ does not render the determination of that question easy 

 The placenta is the cellular expansion by which the seed is attached to the ovary, 

 and brought into direct connexion with the sexual organs of the plant through its pro- 

 longation the conducting tissue of the style. 



