EMBRYOLOGY 



359 



other parts originate from the epibasal tier : the cotyledon with its ligule 

 is the first part to be organised : it is followed soon by the first root, 

 which arises at the opposite side of the epibasal tier to the cotyledon 

 (Fig. 191 c, D, E, F). Between these parts a slight depression is formed, 

 and it is surrounded by a semicircular ridge : within this the apex of the 

 axis is at last organised, and it soon gives rise to the second leaf, which 



St.,' 



FIG. 191. 



Isoetcs echinospora, var. Braunti, Dur. X 365. /4=an archegonium. B = a. two- 

 celled embryo within the archegonium. D, E, F= three successive horizontal sections 

 of a somewhat advanced embryo; /? = root; cot cotyledon ; st-^t&m ; /=ligule. x 175. 

 C = median longitudinal section of a young embryo; /=ligule. X 200. G = median 

 section of a young sporophyte with second leaf, h, already formed; r 2 = second root; 

 *^ = stem apex, x 150. (After Campbell.) 



faces the cotyledon : the very exiguous apical cone lies between them 

 (Fig. 191 G). And so the shoot is established, bearing successive leaves 

 with spiral arrangement, and successive roots, of which the second is below 

 the base of the second leaf. 



Comparing this apparently divergent embryogeny with that of the 

 Lycopods above described, if the whole embryo be imagined inverted 

 in its orientation relative to the archegonial neck, and their suspensor 

 be imagined entirely away, then the two embryonic tiers may be com- 



