REPRODUCTION. 631 



In an "exalbuminous" seed, only two generations can 

 be traced ; 



Integuments = tissue of parent-sporophore 



Embryo = the new sporophore. 



There is also this further peculiarity of Phanerogams 

 to be noted, that, whereas in all other cases the development 

 of the embryo from the sexually produced spore goes on 

 continuously until the adult form is reached, in the Phane- 

 rogams the development of the embryo is discontinuous. It 

 takes place, namely, in two stages. The first of these, as 

 mentioned above, terminates with the ripening of the seed ; 

 the second begins with what is known as the germination 

 of the seed, and includes the escape of the embryo from 

 the seed and the gradual attainment of the adult form; 

 between these two periods there intervenes a longer or shorter 

 period of quiescence. The degree of development which 

 may be reached by the embryo within the seed is different in 

 different cases; in an albuminous seed the embryo is small, 

 occupying but a portion of the embryo-sac ; in an exalbumi- 

 nous seed the embryo-sac, to begin with, occupies the whole 

 of the interior of the seed, and the embryo entirely fills 

 the embryo-sac (see p. 179). 



Returning now from this digression, we find that in the 

 life-history of the Mosses and of the plants above them in the 

 scale of organisation, there is a regular alternation of gene- 

 rations of such a kind, that twice in the life-history the plant 

 is represented by a single cell, a spore, which in the one case 

 has been produced asexually, in the other sexually; the 

 asexually produced spore gives rise to the sexual generation; 

 the sexual generation produces the sexually produced spore; 

 the sexually produced spore gives rise to the asexual gene- 

 ration, which again produces spores asexually. 



We have now to consider how far the life-histories of 

 plants lower than the Mosses conform to this type. Be- 

 ginning with the Algae, and confining our attention to those 

 plants which have distinct sexual and asexual forms, we find 

 that in some, the Volvocineae for example, no alternation 



