REPRODUCTION IN PLANTS 



261 



in the production of a small, green, flat, somewhat heart-shaped structure 

 known as the prothallus, which constitutes the gametophyte generation. 

 Except for the region of the marginal notch, the prothallus is but one 

 cell thick. It produces a number of threadlike structures (rhizoids) on 

 its lower surface, which extend into the soil, where they take up water and 

 salts. 



Sexual reproduction. The sex organs are borne on the lower surface 

 of the prothallus. The female sex organs, or archegonia, are flask-shaped 

 and are borne in the region of the thickened marginal notch. Each 

 archegonium contains a single egg at the bottom of the open-mouthed 



Fig. 17.7. The gametophyte generation of the fern. Photomicrographs of slide preparations 

 of fern prothallus, showing at left the part bearing the antheridia, at right the part bearing 

 the archegonia. (Courtesy General Biological Supply House, Inc.) 



flask. The male sex organs, or antheridia, are more or less spherical in 

 shape and are smaller than the archegonia; they are located near the 

 apex of the "heart." A number of sperms are formed within each antheri- 

 dium. When a ripened antheridium becomes wet with rain or dew, it 

 swells and bursts, thus allowing the sperms to escape. The latter swim 

 through the water film to the archegonia, which they enter by way of the 

 open mouths. Within an archegonium, a single sperm fuses with the egg 

 to form a zygote. 



The development of the sporophyte. The zygote begins its develop- 

 ment within the walls of the archegonium and soon forms definite struc- 

 tures of its own: a "foot" that enables the developing embryo to absorb 

 nourishment from the prothallus, and an embryonic root, stem, and 

 leaves. When the latter have become functional, the prothallus shrivels, 

 and the young sporophyte is left dependent upon its own tissues for 

 maintenance. 



