THE BRYOPHYTA: MUSCI, THE MOSSES 433 



systems. By the term homologous we mean that a certain structure is of 

 the same botanical nature as another similar structure or has been evolved 

 from a common ancestral structure. Since the Moss plant is a gametophyte, 

 and the higher plant with which we are comparing it is a sporophyte, it is 

 obvious that these svstems cannot be of identical nature. 



Sexual Reproduction 



The antheridia and archegonia arise at the top of the leafy stems within 

 an involucre of leaves as in Mnium, though here the leaves resemble the 

 vegetative leaves, except that those of the male are reddish in colour. In 

 the male plant, however, the apex of the stem terminates in a vegetative bud 

 in the middle of the antheridial head, which, after the development of the 

 antheridia, may grow out in the following year and produce a fresh shoot, 

 at the apex of which more antheridia may be produced. This proliferation 

 through the antheridial head may be repeated several times. 



Leptoids 



Endodermis 



Central cylinder 



Hypodermal 

 strand 



Fig. 420. — Polytrichum corumune. Transverse section 

 of rhizome bearing rhizoids. 



The involucral leaves in the male head are not confined to the margin 

 as they are in Mnium, but are arranged spirally from the vegetative apex 

 outwards. The antheridia are produced in groups in the axils of these 

 leaves so that the whole head is really compound and contains a variable 

 number of these closely set antheridial groups. 



Since the development of the antheridia and archegonia in Polytrichum 

 is essentiallv similar to that in Mnium it will not be necessarv' for us to consider 

 the stages in detail (Figs. 421 and 422). 



15 



