THE EVOLUTION OF PLANT REPRODUCTION 57 



(therefore unable to carry on autotrophic metaboHsm), and 

 grows up from the leafy green moss plant as a parasite. When 

 mature, it produces a capsule at its tip in which many thousands 

 of small spores are produced. Each spore, carried to a new 

 environment by the wind, germinates into another leafy green 

 moss plant like its grandparent (fig. 30). 



Such a cycle of asexual and sexual reproduction is known as 

 alteration of generations, and the sexual plant is referred to as a 

 GAMETOPHYTE, the ascxual plant as a sporophyte. In the life 

 history of any plant going through alternation of generations, a 

 spore always develops into a gametophyte while a fertilized egg 

 grows into a sporophyte. In the moss, dispersal of new plants 

 occurs during the asexual reproductive phase with the scattering 

 of the spores. 



The Reproductive Cycle of a Fern 



This alternation of a sporophyte with a gametophyte is 

 carried out with increasing specialization of the sporophyte, in 

 the FERNS. The familiar leafy fern plant begins its reproductive 

 phase by the development of sporangia, either on the under side 

 of the leaves or on special parts of the plant (fig. 31). Each spo- 

 rangium consists of a stalk and a spherical capsule filled with 

 spores. Since the fern reproduces asexually in this way, unlike the 

 leafy moss plant, it is a sporophyte. 



After a spore has fallen to the ground, it germinates into a 

 new fern generation which is as different in appearance from the 

 parent fern plant, as the new moss generation is from its parent. 

 This fern plant, developing from the spore, is a gametophyte 

 known as a prothallus — a flattened and heart-shaped plant 

 attached to the ground by tufts of rhizoids on its under surface 

 (fig. 32). Among the rhizoids are the archegonia and anther- 

 idia, the former flask-shaped with necks protruding beyond the 

 prothallus, the latter barrel-shaped structures made up of rela- 

 tively few cells. When there is sufficient moisture, the sperm cells 

 swim to the archegonia and fertilize the eggs. Like the fertilized 

 Q^g of the moss gametophyte, that of the fern does not grow into 

 another plant like its immediate parent. Instead, remaining in 

 place, the fertilized ^g'g develops into an embryo which soon 



