THE PLANT KINGDOM 251 



this spore-plant will germinate and grow (if conditions arc 

 favorable) and will develop in turn into a new gamete-plant, 

 which will produce gametes and effect fertilization as before. 

 In such a life-cycle as this, there are two distinct, independent 

 and alternating plant types or "generations," each always 

 producing the other. It was a study of conditions in these 

 Pteridophytes which led to the term "alternation of generations", 

 and which caused us to realize the significance of the peculiar 

 methods of reproduction found in both the mosses and the 

 seed plants. 



Another notable distinction between gametophytc and sporo- 

 phyte lies in the number of chromosomes found in their cells. 

 In the former this number is only half as great as in the latter. 

 The gametophyte really begins at the "reduction division" 

 (p. 187 ), which occurs in one of the cell divisions preceding the 

 formation of the spores, and ends with the union of the gametes 

 in fertilization, which restores the double chromosome number and 

 begins the sporophyte. 



D. In Sperrnatophytes. — Finally, in the fourth and highest 

 division of the plant kingdom, the Spermatophytes or seed plants, 

 the alternation of generations has reached a still further stage of 

 specialization. Here the gametophyte, instead of being an 

 independent structure, now remains attached to and dependent 

 upon the sporophyte. Furthermore there are now two kinds of 

 spores, the microspores (essentially what we know as pollen 

 grains) which develop into much reduced male gametophytes, 

 producing only male gametes; and the megaspores, borne in the 

 ovules, and developing there into female gametophytes, producing 

 only egg cells.* At maturity, the male gamete comes down the 

 pollen tube and fertilizes the egg in an ovule. From this union 

 the young sporophyte develops as the embryo of the seed, and 

 will in turn grow into a plant producing thousands of spores. 

 In the seed plants, both gametophytes are much reduced in size 

 and have so lost their primitive character that it was long 

 before they were recognized as gametophytes at all. 



In the history of the plant kingdom we thus pass from plants 

 which are, like animals, entirely gamete-bearing (in the Thallo- 



* Microspores and megaspores are differentiated in a few of the higher 

 Pteridophytes, which we shall later describe. No seeds are developed among 

 these plants, however. 



