STUDIES OF SPOKE-PLANTS 239 



formed without sex-organs ; that is, by asexual reproduction. 

 The prothallium reproduces the fern plant by sex-cells 

 (egg-cells and sperm-cells). The two are necessary to make 

 a complete life-history, for the spores cannot develop directly 

 into a fern plant and the fertilized egg-cell cannot develop 

 directly into a new prothallium. The prothallium and the 

 fern plant each represents what is known in biology as a 

 generation, and the succeeding each other is alternation of 

 generations. It should be especially noted that a generation 

 reproducing asexually (i.e., the fern plant) alternates with 

 the one which reproduces by the sexual method (i.e., the 

 prothallium) . 



The asexual generation represented by the fern-plant is 

 often known as the sporophyte (meaning spore-plant, because 

 it forms spores) ; while the sexual generation, the prothal- 

 lium, is known as gametophyte (meaning gamete-plant, 

 because it develops the gametes or male and female sex-cells). 



It is interesting to note that the spore-generation, the fern 

 plant, may live very many years and reach a large size (some 

 tree-ferns of the tropics are forty or more feet high). The 

 sex-generation, the prothallium, is always very small and lives 

 only a few months. In fact, it is so small and attracts so 

 little attention that only by most careful studies were the 

 facts concerning alternation of two generations discovered. 



By life-history, or life-cycle, of a fern we understand all 

 stages from spore to spore again ; that is, spore, prothallium, 

 sex-cells, fertilized egg-cell, embryo, fern-plant which forms 

 spores starting a new life-history. 



227. Alternation of Generations in Seed-plants. The 

 similarity of structure between roots, stems, and leaves of 

 ferns and the lowest seed-plants suggests that they are 

 related. The fact that the ferns have alternation of two 

 generations (prothallium and fern plant) makes their life- 

 history appear at first to be entirely different from seed- 

 plants; but careful investigations have shown that even 



