422 A TEXTBOOK OF THEORETICAL BOTANY 



upwards. This cell then divides by a transverse wall cutting off a central 

 cell and an upper cap cell. From the peripheral cells the wall of the arche- 

 gonium is formed, whilst the central cell gives rise to the oosphere and 

 the cells of the neck canal. It will be seen, then, that the development of 

 the archegonium in Mnium is essentially similar to that in Pellia. Scattered 

 among the archegonia are numerous paraphyses similar to those found in the 

 male plants. These paraphyses are composed of a single row of cells which 

 gradually enlarge towards their apex. Their walls are often brown in colour, 

 especially in the lower part, but all the cells contain chloroplasts. 



Fertilization is effected in the same way as in the Liverworts. It usually 

 occurs in May. The antherozoids migrate from the antheridia of the male 

 plant to the top of the neck of the archegonium and make their way down, 

 but one only fuses with the oosphere. We have already discussed in our 

 consideration of Marchantia the means by which the antherozoids are 

 enabled to make this journey. In Mtiium it is even more difficult to see how 

 the extremely delicate antherozoid contrives to make its way from the male 

 plant to the female. Nevertheless from the number of capsules which are 

 produced on any clump of Moss we conclude that no insuperable difficulty 

 is encountered. Presumably fertilization can only take place when the Moss 

 plants are well covered with moisture, to which the paraphyses contribute 

 by water secretion, but it is worth while to consider the relative simplicity 

 of fertilization among the aquatic Thallophyta and the difficulty which must 

 have been experienced by plants as soon as they migrated to the land. 



Development of the Sporogonium 



Though more than one archegonium may be fertilized, normally only 

 one sporogonium develops on each shoot of the female Moss plant. Although 

 fertilization occurs in May, the development of the sporogonium is not 

 complete until April of the following year. 



The oospore divides into two by a transverse wall, and each cell then 

 divides further so as to produce a spindle-shaped structure of small-celled 

 tissue, with a large cell at either end (Fig. 406). From one end of this tissue 

 the foot develops, by means of which the embryo is anchored into the tissue 

 at the base of the archegonium. The seta or stalk of the sporogonium 

 develops from the lower middle cells of the embryo, whilst the capsule is 

 formed from the whole of the upper cells. As growth continues the foot 

 makes its way downwards through the receptacle of the gametophyte and 

 comes into contact with the top of the axial strand of conducting tissue in 

 its stem. Although no cellular fusion occurs between the two tissues it will 

 be realized that in this position it is enabled to obtain the maximum amount 

 of water and other materials passing up the conducting cells. 



For some time the seta remains quite short, but when it increases in 

 length the body of the archegonium also continues to grow, and for a time 

 keeps pace with the development of the sporogonium and acts as a cover 

 to it. The neck of the archegonium shrivels, but when eventually the body 



