POSITION OF THE EMBRYO 133 



The Radicle. — The Radicle (n in the last-named figures) is the extreme 

 tip of the caulicle, which points always in the direction of the micropyle. 

 From this point the root is to be developed. The embryo may consist 

 of nothing further than the caulicle, and even this may be of the most 

 elementary character. 



The Cotyledons. — Ordinarily, however, there develops at the node 

 (the point opposite to the radicle) one or more Cotyledons, or Seed- 

 leaves (cot in the figures). 



Monocotyledons, Dicotyledons, and Polycotyledons. — Most seeds which 

 possess but a single cotyledon (Fig. 404) are grouped together in a 

 division of the Angiosperms, which for this reason are called Mono- 

 cotyledons, those with two in the Dicotyledons. A few plants, mostly 

 Gymnosperms, are polycotyledons (Fig. 406). 



The Plumule. — The highest plants of their respective groups develop 

 a second phytomer lying between the cotyledons, or if there be but one 

 cotyledon, mostly enwrapped by it. This is the Plumule (Fig. 405, pi), 

 which shows the same variation in the degree of its development as 

 that which characterizes the lower. When its leaves are developed, 

 they bear a closer resemblance, as in the figure, to the mature leaves 

 of the plant than do the cotyledons, following out the law referred to in 

 our introduction. Among dicotyledons, the plumule commonly pertains 

 to exalbuminous seeds. 



Direction of the Radicle. — Terms used to indicate difl^erent directions 

 of the radicle refer to its direction with relation to the fruit, its direction 

 in relation to the micropyle being, as has been stated, always the same. 

 It is Ascending when it points toward the apex of the fruit. Descending 

 when in the opposite direction, and Horizontal when intermediate. 

 The latter form is Centrifugal when pointing toward the periphery, 

 Centripetal when toward the axis. 



Position of the Embryo. — The position of the embryo with reference 

 to the albumin is always highly characteristic. It is Axile or Centric 

 when in the center of the albumin (Figs. 402 and 403), whether straight 

 or curved; Eccentric when within the albumin, but outside of its center 

 (Fig. 407) ; Peripheral when lying upon the surface of the albumin. In 

 the latter position, it may be straight, simply curved (Fig. 408), or 

 circinately coiled (Figs. 409 and 410). 



The relative sizes of the embryo and the albumin vary from those 

 in which the former is a mere speck in a large mass of the latter to that 

 in which the proportions are reversed, or in which the albumin is 

 entirely wanting. 



