2 PART I. MORPHOLOGY. [ 2 



both in the varietj^ of form presented by different plants, and in the 

 varioias stages in the development of any one individual plant. 



2. The Life-History of Plants. The consideration of this 

 subject is a necessary preliminary to the detailed study of 

 Morphology. The great majority of plants are more or less poly- 

 morphic : that is, the plant assumes, as a rule, at least two quite 

 different forms in the course of its life. Most commonly it presents 

 but two forms which, while they may differ more or less widely in 

 form and structure, are essentially distinguished by the fact that 

 the one, termed the sporophyte, has asexual reproductive organs 

 which produce asexual reproductive cells, termed spores, each of 

 which is capable by itself of giving rise to a new organism ; whilst 

 the other, termed the gametophyte, has sexual reproductive organs, 

 which produce sexual reproductive cells, termed gametes, and 

 though each of these cells is by itself incapable of giving rise to 

 a new organism, yet by the fusion of two of these gametes of 

 different sex, a cell is formed which is of the nature of a spore, 

 since from it a new organism can be developed. These two forms 

 alternate more or less regularly in different plants, the asexually- 

 produced spore of the sporophyte giving rise to a gametophyte ; the 

 sexually-produced spore of the gametophyte giving rise to a 

 sporophyte. Such a life-history presents what is known as alter- 

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

 asexual form. 



The alternation of generations is conspicuous in the Bryophyta 

 and the Pteridophyta, as is fully explained in the chapters 

 specially devoted to those groups. It also occurs in the life- 

 history of the Phanerogams, and may be traced, more or less 

 imperfectly, in some of the Thallophyta. But since the tracing of 

 it in the last-named group is attended with some uncertainty, that 

 group will be excluded from further consideration here. In the 

 groups Bryophyta, Pteridophyta, and Phanerogamia, the two 

 generations attain very different degrees of development. In the 

 Bryophyta, the gametophyte is the more conspicuous generation ; 

 it is the form to which the name attaches, and upon which the 

 classification is mainly based ; whereas, the sporophyte is, as it 

 were, an appendage to the gametophyte, and is generally known as 

 the Moss-fruit. In the Pteridophyta, the sporophyte is the con- 

 spicuous form to which the name of the plant attaches; but, 

 though small and inconspicuous, the gametophyte is an independent 

 organism known as the prothallus. In the Phanerogamia, as in 



