THE EVOLUTION OF THE SPOROPHYTE. 



with the much simpler oogonium, or female organ of the green 

 algae, and its origin is still a matter of conjecture. 



Aside from the character of the sexual organs, there is no 

 difficulty in homologizing the sexual plant (gametophyte) of the 

 lower liverworts and that of certain green algae. 



As a result of the change from the aquatic to an aerial envi- 

 ronment, we find the tissues of the gametophyte in the liver- 

 worts more resistant to loss of water than is the case in the 

 algae. The outer cell-walls are cuticularized, and there is 

 more or less specialization 

 of the inner tissues, and this 

 specialization may be very 

 pronounced, as we find in 

 such large liverworts as 

 Marchantia. Finally, in the 

 leafy liverworts and true 

 mosses, the gametophyte 

 develops a definite axis and 

 special assimilating organs, 

 leaves. 



It is the sporophyte, how- 

 ever, or non-sexual plant, 

 developed from the fertilized 

 ^^^ within the archegonium, 

 that we wish especially to consider. We have already intimated 

 that in certain algse the oospore, on germination, gives rise to a 

 globular cell-mass (Fig. i, a), each cell of which produces a zoo- 

 spore. In the lowest liverworts, e.g., Riccia (Fig. i, <^), a similar 

 globular cell-mass is produced from the fertilized ^^g, but there 

 is a much shorter period intervening between fertilization and 

 the germination of the spore. This cell-mass, while directly 

 comparable to the corresponding structure in Coleochaete, 

 differs from it in two important particulars. In the first place, 

 a single external layer of cells in the sporophyte of Riccia 

 (Fig. 3, a) remains sterile, and, secondly, each cell of the interior 

 sporogenous tissues, instead of producing a single zoospore, 

 gives rise to four non-motile, thick-walled spores, which do not 

 germinate immediately, but are capable of becoming quite dry 



Fig. 2. — a, The archegonium, or female organ, of a 

 liverwort (Targionia), seen in longitudinal section. 

 b, A similar section of the archegonium of the cin- 

 namon fern ; o, the egg-cell. 



