CH. VI MOSSES {MUSCJ): SPHAGNACE^E— ANDRE ALACEjE 153 



Hypnuvi ; others grow regularly in very exposed situations on 

 rocks, e.g. Andreaa. Very many, like Funaria Jiygromctrica and 

 Atrichuin undulatitm, grow upon the earth ; and others again, like 

 species of Mnium and TJmidiuni, seem to grow exclusively upon 

 the decaying trunks of trees. Indeed Mosses are hardly absent 

 from any locality except salt water. With the exception of the 

 Sphagnaceae and Andreaeacese, and possibly Archidiuin, the type 

 of structure found among the Mosses is extraordinarily constant, 

 and they may all be unhesitatingly referred to a single order, 

 the Bryaceas, which includes within it an overwhelming majority 

 of the species. 



The gametophyte of the Musci always shows a well-marked 

 protonema, which in most cases has the form of an extensively 

 branching alga-like filamentous structure, from which later a 

 distinct leafy axis arises as a lateral bud. In Sphagnum this 

 protonema is a flat thallus, and the same is true of TetrapJiis 

 and a few other forms, but the filamentous protonema is very 

 much more common. The gametophore arises from this 

 protonema as a lateral bud, which develops a pyramidal apical 

 cell, from which three sets of segments are cut off, each segment 

 developing a leaf. The only exception to this, so far as is 

 known at present, is the genus Fissidens} where the apical cell 

 is wedge-shaped, and only two sets of segments are formed. 

 Upon these leafy branches the sexual organs are borne. The 

 relative degree of development of the protonema and the game- 

 tophore differ much in different forms. Thus in the Phascaceae 

 the protonema is permanent, and the gametophore small and 

 poorly developed. In the higher ones, the protonema dis- 

 appears more or less completely, and the assimilative functions 

 are entirely assumed by the large highly developed gametophore, 

 which is capable of reproducing itself by direct branching 

 without the intervention of the protonema. The commonest 

 form of gametophore is the upright stem with the leaves 

 arranged radially about it, but in many creeping forms, such as 

 some species of Mniujii, Hypnuin, etc., the gametophore is more 

 or less dorsiventral ; but in these the apical cell is pyramidal, 

 and produces three rows of leaves. Growing out from the base 

 of the stem in most Mosses, and fastening it to the substratum, 

 are numerous brown rhizoids which are not, however, morpho- 

 logically distinct from the protonema. Thus if a turf of growing 



^ Leitgeb (2). 



