March, 1906.] Life Cycle of a Homosporous Pteridophyte. 487 



pendent when the gametophyte dies. When normal conditions 

 are present the development of the embryo always results in the 

 death of the parent gametophyte. 



During the juvenile stage the young sporophyte usually has 

 a type of leaves different from those of the mature condition, but 

 gradually it becomer more and more like the mature fern and 

 finally takes on the form normal to the species, in which condi- 

 tion it may live to an indefinite age. The horizontal rhizome 

 continues to develop and branch at the tip and if decay takes 

 place at the back end vegetative propagation is accomplished 

 and the result is a larger and larger number of independent 

 individuals. 



The four main stages of a fern are therefore as follows: 



1. The sporophyte or fern plant proper. 



2. The nonsexual spores produced as the result of a reduc- 

 tion division. 



3. The gametophyte or thallus plant. 



4. The oospore produced as the result of the conjugation of 

 the egg and sperm. 



An interesting deviation from the usual type of alternation of 

 generations is present in some ferns. In a few species a thallus 

 may develop by vegetative propagation from the tissues of the 

 sporophyte. This is called apospory. Or the sporophyte may 

 develop directly from the tissues of the gametophyte and not 

 from an egg. This is known as apogamy. The details of 

 apogamous and aposporous structures have not been investi- 

 gated to any great extent and there is still doubt as to whether 

 or not a conjugation or a reduction takes place. 



But if these important processes are omitted the change from 

 gametophytic to sporophytic characters or vice versa may be 

 explained in the following manner. It is evident that ever}' cell 

 in the gametophyte, or at the least the reproductive ones, must 

 also contain the hereditary characters which are present in the 

 sporophyte; but these characters are for the time being dormant. 

 In the same way every cell in the sporophyte must also possess 

 the hereditary characters peculiar to the gametophyte. Now, 

 in ordinary cases gametophytic characters become active and 

 assert themselves onh' as the result of a reduction division and 

 sporophytic hereditary tendencies are onl}' apparent after the 

 conjugation of the egg and sperm. But there may be other 

 stimuli able to induce the change, a response to which in the 

 case of apogamy causes the gametophytic hereditary tendencies 

 to become dormant and the sporophytic tendencies to become 

 active, thus producing a sporophytic shoot; or in apospory, 

 causes the sporophytic tendencies to become dormant while the 

 gametophytic tendencies resume activity. 



