342 NATURE STUDY AND AGRICULTURE 



lying prostrate on muddy flats; such flats as we find on very 

 gently sloping seashores between the tide marks. Swim- 

 ming spores were necessarily abandoned, and light spores 

 for dispersal by air took their place. The gametes 

 (sperms and eggs) were produced in better protected 

 organs, with jackets of protecting cells. Such an organ 

 containing sperms is the antheridium, and the one contain- 

 ing eggs is the archegonium. 



However, the greatest change introduced at this stage 

 of evolution was what is called the alternation oj genera- 

 tions, and this change profoundly affected all the future 

 development of the plant kingdom. Unless it is under- 

 stood, no true interpretation of plant-life histories is possi- 

 ble. In this early alternation of generations the green 

 plant body formed no spores, but did form the sperms and 

 eggs. Then the fertilized eggs, instead of producing a form 

 like their parent, as animal eggs do, produce an entirely 

 different structure. This other structure, or other plant, 

 produces no sperms and eggs, but does produce spores; 

 the plant is thus for the second time, but in a very different 

 way, reduced to the one-cell stage in its life history. Then 

 when one of these one-celled spores germinates (which it 

 does of course without waiting for any act of fertilization), 

 it produces the original sperm-and-egg-bearing plant. 

 In this way two really perfectly distinct plants alternate 

 with each other in making up the life history. The sperm- 

 and-egg producing generation is called the gametophyte or 

 gamete plant, and the other the sporophyte or spore plant. 

 The gametophyte is sexual, and the sporophyte is sexless, 

 and each in turn produces the other, and this habit con- 

 tinues throughout all the higher groups of plants. 



