CH. XI] THE MOSS PLANTS 479 



acteristic of this group and the Pteridophytes), is fertilized 

 by a free-swimming sperm cell formed in an elongated 

 many-celled ANTHERIDIUM. Thus the fertilization requires 

 water (in this case rain or dew) as in the Algae, from which 

 the method is a survival. The fertilized egg cell grows at 

 once into a more or less elaborate SPOROGONIUM or spore 

 capsule, containing numerous asexual spores, which are 

 disseminated by wind; and these spores develop into new 

 plants. It is perfectly clear that the Moss or Liverwort 

 plant represents the sexual generation, or GAMETOPHYTE, while 

 the spore capsule with its appurtenances represents the 

 asexual generation, or SPOROPHYTE. With these Bryophytes 

 this alternation of generations, of which anticipations oc- 

 curred in the Algse and Fungi (pages 423, 446, 449), becomes 

 firmly fixed, and it persists thenceforth throughout the 

 higher plants. Moreover, this alternation bears in these 

 plants a very definite relation to the reduction division of 

 the chromosomes (page 285), for the number of chromosomes 

 is always reduced in the divisions concerned in the develop- 

 ment of spores, which are regularly formed in groups of four. 

 The reduced number persists through all of the cells of the 

 gametophyte, and is doubled with fertilization, after which 

 the full number occurs in all of the cells of the sporophyte 

 until the formation of the spores. It seems reasonably clear 

 that the alternation of generations had originally the sig- 

 nificance of a method of securing the development of several 

 individuals from one act of fertilization. Later it became 

 involved in alternation between fertilization and the reduction 

 division, and thus it persists in the higher plants. 



The Bryophytes fall into two distinct classes as follows : 



CLASS 1. HEPATIC.^E : THE LIVERWORTS. 

 CLASS 2. Musci : THE MOSSES. 



CLASS 1. HEPATIC.*:: THE LIVERWORTS 



These are typically thallose land plants growing flat on 

 the ground in moist places, though some float on water, or 



