592 



PHYSIOLOGY. 



simultaneously with it, the two seed-leaves or cotyledons (p. 156) 

 gradually emerge from the seed-coats, being raised by the growth 

 upwards of the tigellum, or hypocotyledonary axis (fig. 596). If the 

 seed-leaves are thin and leafy they are pushed up above ground, and 

 become epic/eal, fulfilling at once the functions of true leaves. If 

 they are thick and fleshy, they generally remain beneath the soil 

 (hypogeal}, or, at any rate, do not perform the functions of leaves 

 but only of store-houses (fig. 596). Between the two cotyledons, 

 at the summit of the tigellum, may be seen the plumule or rudimen- 

 tary bud enclosing the growing point by means of which the stem 

 elongates. Axillary buds, each with its growing point, are some- 

 times produced in the axils of the cotyledons. 



The general process of germination in Dicotyledons is subject to various 

 modifications, such as the union of the two cotyledons in Anemone, 

 Acom'tum, &c., the growing point then making its way through a slit or 

 aperture in the stalks of the cotyledons, which latter, in such cases, often 

 remain within the seed. In Cyclamen there is but a single cotyledon, 

 while in other cases no trace at all of cotyledons is visible, the embryo con- 

 sisting then of a thick tigellum, one end of which tapers ino a radicle, 

 the other forms a stem as in some Myrtaceae and Clusiaceae. 



Fig. 596. 



Germination of the Bean. 



I. Seed; the black mark at the lower end is the hilum. 



II. Outer surface of embryo, the seed-coat removed, c, cotyledon, r, radicle. 



III. Cotyledon separated from its fellow and seen from within, p, plumule ; r, radicle, 



IV. Shows protrusion of radicle and plumule ; the cotyledons still partly enclosed within 

 the seed. 



The development of the so-called premorse root and the formation of 



