PTEKIDOPHYTES 



309 



or less spherical structures distributed among the rhizoids at the 

 posterior end of the plant. This position of the sex organs next 

 to the moist soil is therefore favorable to fertilization \)j the mo- 

 tile male gametes of the fern. Fertilization is always preceded, 

 as in the moss, by the expulsion of the male gametes during 

 wet weather or when the plants are covered with dew, and by 

 the transformation of the canal cells of the archegonium into a 

 mucilaginous substance which attracts the sperms to the eggs. 



A 



Rhizome 

 c 



FIG. 178. Gametophyte and embryo of Adiantum 



A, garaetophyte and young attached sporophyte of Adiantum ; B, section of gameto- 

 phyte and embryo of a fern : a, young embryo ; 6, more mature embryo with first 

 leaf ; c, permanent rhizome, leaves, and roots, which grow from the stem portion of 

 the embryo in b. The embryonic root, foot, and leaf of the embryo in b disappear 

 when the plant in c is fully established 



Embryo. After fertilization, while still in the venter of 

 the archegonium, the zygote germinates to form the embryo 

 (Fig. 178, B, a). The young sporophyte, quite unlike that of 

 the moss, very early differentiates into a primary root, stem, 

 and cotyledon, which indicates that it is to become a real leafy 

 plant with the usual organs of the higher plants (Fig. 178 5, 6). 

 In addition to these primary organs the embryo spore plant 

 develops a mass of cells called the foot, which remains in con- 

 tact with the gametophyte plant and absorbs nourishment from 

 it for the growing embryo until the latter is self supporting (c) 

 and the gametophyte dies. 



