FERN STRUCTURE 255 



structures consisting of an outer layer of cells surrounding 

 a central mass of small cells, each of which produces 

 a sperm. When mature, the antherids rupture and 

 permit the escape of the spiral multiciliated sperms 

 which swim with a rotary motion. 



452. The archegones are flask-shaped organs sunken 

 into the tissues of the plant. At first 



the neck is closed, but at maturity it 

 opens down to the egg. Fertilization 

 takes place in water (after rains or 

 heavy dews), the sperms swimming 

 to and down the neck of the arche- F^ 131. Fem arch- 



, r , i . , ... egone, egg, antherid and 



gone, where one of them unites with sperm. 

 the egg. 



453. Sporophyte. After fertilization the egg divides 

 again and again, soon producing a solid stem from which 

 a root springs at one end, while from the other the leaves 

 arise. The latter are at first small and quite simple in 

 structure, but those formed later are larger and more 

 and more complex in structure, until finally the full form 



is reached, and still later the full 

 size. The stem, bearing leaves 

 and roots, constitutes the sporo- 

 phyte, which is sharply contrasted 

 with the gametophyte in structure, 

 FIG. 132. Development of size, and duration, the latter being 



fern sporophyte. . 



short-lived, small, and of simple 



structure, while the former is long-lived, often of large 

 size, and of great complexity of structure. On this 

 plant the spores are eventually produced which on 

 germination give rise to gametophytes like those with 

 which we started, thus completing the round of life. In 

 most Ferns the spores are of one kind, only (isospores), 

 but in a few they are of two kinds (heterospores) in 



