BIOLOGICAL LECTURES. 



lower liverworts, which represent the simplest types of existing 

 land plants, probably originated in some such way. There are 

 still existing certain amphibious liverworts, species of Riccia, 

 which live for the most part as floating aquatics, but sometimes 

 settle down in the mud left by the subsiding water, or even 

 creep up the muddy banks and establish themselves as land 

 plants. This seems to be most commonly done before the devel- 

 opment of the reproductive organs. It is very likely that the 

 individual history of these liverworts illustrates the origin of 



the typical forms from their 

 aquatic ancestors. 



Among those algae which 

 approach most nearly to the 

 lower liverworts there is a 

 hint of the characteristic al- 

 ternation of generations so 

 conspicuous in the mosses 

 and ferns. The egg-cell in 

 such algae, on being fertil- 

 ized, produces a resting- 

 spore which may be invested 

 with a protective envelope 

 of cells. This resting-spore 

 (oospore) does not, on germination, produce a new plant at 

 once, but a globular cellular mass is formed (Fig. i, a), each 

 cell of which gives rise to a motile zoospore, which then grows 

 into a plant like the parent one. 



This alternation of sexual and non-sexual plants becomes, 

 as is well known, very prominent in the mosses and ferns, or 

 Archegoniates as they are called on account of the peculiar 

 female organ, the archegonium. 



While there is unmistakable evidence of relationship between 

 the lower liverworts and the green algae, it must be admitted 

 that at present the gap between the two groups is a very great 

 one, and we are not in a position to state positively just where 

 is the point of contact between Archegoniates and algae. 



The structure of the archegonium (Fig. 2) is very constant 

 throughout the Archegoniates, but has very little in common 



Fig. I . — a, Diagram of a section through the germi- 

 nated resting-spore of an alga (Coleochaete), showing 

 the mass of cells within. Each cell of the rudi- 

 mentary sporophyte produces a single zoospore. 

 b, Longitudinal section of the lower part of the 

 fertilized archegonium of a liverwort (Riccia), 

 showing the contained sporophyte, sp., developed 

 from the egg-cell. Each cell of the sporophyte, 

 except the outside ones, gives rise later to four 

 non-motile spores. 



