138 PLANT STRUCTURES 



is in connection with the asexual spores, which may be 

 kept for a long time and then germinated. 



The first step in germination is for the oospore to di- 

 vide into two cells, forming a two-celled embryo. In the 

 ordinary Ferns this first dividing wall is at right angles to 

 the surface of the prothallium, and is called the basal wall 

 (Fig. 119, A). One of the two cells, therefore, is anterior 

 (toward the notch of the prothallium), and the other is 

 posterior. 



The two cells next divide by forming walls at right 

 angles to the basal wall, and a four-celled embryo is the 

 result. This is called the " quadrant stage" of the em- 

 bryo, as each one of the four cells is like the quadrant of a 

 sphere. 



With the appearance of the quadrant, four body regions 

 are organized, each cell by its subsequent divisions giving 

 rise to a distinct working region (Fig. 119, A). Two of the 

 cells are inner (away from the substratum) ; also one of the 

 inner and one of the outer (toward the substratum) cells 

 are anterior ; while the two other inner and outer cells are 

 posterior. The anterior outer cell develops the first leaf of 

 the embryo, generally called the cotyledon (Fig. 119, b) ; the 

 anterior inner cell develops the stem (Fig. 119, s) ; the pos- 

 terior outer cell develops the first (primary) root (Fig. 

 119, w) ; the posterior inner cell develops a special organ 

 for the use of the embryo, called the foot (Fig. 119, /). 

 The foot remains in close contact with the prothallium and 

 absorbs nourishment from it for the young embryo. When 

 the young sporophyte has developed enough to become in- 

 dependent the foot disappears. It is therefore spoken of 

 as a temporary organ of the embryo. It is necessary for the 

 leaf to emerge from beneath the prothallium, and it may 

 be seen usually curving upward through the notch. The 

 other parts remain subterranean. 



(2) The root. The primary root organized by one of 

 the quadrants of the embryo is a temporary affair (Figs. 



