INTRODUCTORY. 3 



pairs form cells, of the nature of spores, from each of which a 

 new organism can be developed. These two forms alternate more 

 or less regularly in different plants, the asexually-produced 

 spores of the sporophyte giving rise to gametophytes, the sexually- 

 produced spores of the gametophyte giving rise to sporophytes. 

 Such a plant is dimorjjhic, and its life-history presents an alter- 

 nation of generations, that is, an alternation of a sexual with an 

 asexual form. 



A typical instance of such alternation of generations is afforded by the life- 

 history of a Moss. Tbe sexual generation (gametophyte) is the moss-plant 

 bearing the sexual reproductive organs, male (antheridium), and female 

 (archegonium), of which the former produces male reproductive cells (spermato- 

 zoids), the latter a single female cell (oosphere). As the result of the fusion of 

 the male cell with the female cell (sexual process), a cell, the oospore, is produced 

 Avhich is the origin of the asexual generation (sporophyte) known as the moss- 

 sporogonium. The sporogonium produces spores asexually, which, on germina- 

 tion, each give rise to a (at least one) moss-plant (gametophyte). The game- 

 tophyte is thus always developed from a spore produced asexually ; the sporo- 

 phyte, from a spore produced sexually. 



This kind of life-history obtains in the majority of plants, but it may be com- 

 plicated in various ways. Thus, the sexual generation may be represented by 

 two distinct forms, the one male (androphyte), the other female (gynophyte). 

 Again, in some of the lower plants, the gametophyte also gives rise asexually 

 to spore-like reproductive cells (gonidia), which do not, however, enter into the 

 alternation of generations, for they give rise, not to sporophytes, but to repeti- 

 tions of the gametophyte. The sporophyte also may similarly repeat itself, 

 though not always in exactly the same form. Further, it occasionally happens 

 that the gametophyte does not actually produce sexual organs or cells at all, 

 though it is essentially capable of doing so ; and successive generations of such 

 gametophytes may be developed by means of gonidia, until at length one arises 

 which is actually sexual. Such non-sexual gametophytes are designated poten- 

 tial gametopliyfes. 



Without entering into detail, the general morphological rela- 

 tions between the two generations may be briefly considered. In 

 the case of the Moss, the morphological differentiation of the 

 sporophyte is, on the whole, not much lower than that of the 

 gametophyte. In the approach to morphological equality be- 

 tween the two generations, the Mosses occupy a central position 

 in the vegetable kingdom. For in all plants higher in the scale 

 than Mosses, the morphological differentiation of the sporophyte 

 is far higher than that of the gametophyte; whilst in plants lower 

 than the Mosses, the gametophyte is, as a rule, more highly 

 differentiated than the sporophyte. In other words, whereas in 

 the higher plants, the sporophyte is the form to which the name 



