CH.V. MOSSES (MUSCI): SPHAQN ALES ANDREsEALES 161 



Hypnum; others grow regularly in very exposed situations on 

 rocks, e. g., Andrecea. Very many, like Funaria hygronietrica 

 and A trie hum undulatum, grow upon the earth; and others 

 again, like species of Mniuni and Thuidium, 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 Sphagnacese and Andreaeaceae, and pos- 

 sibly Archidium, the type of structure found among the Mosses 

 is extraordinarily constant, and they may all be unhesitatingly 

 referred to a single order, the Bryales, 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 Tetraphis and a few other forms, but the filamentous proto- 

 nema 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 producing a leaf. The only exception 

 to this, so far as is known at present, is the genus Fissidens 

 (Leitgeb (2)), 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 gametophore differ 

 much in different forms. Thus in the Phascaceae the proto- 

 nema is permanent, and the gametophore small and poorly 

 developed. In the higher Mosses the protonema disappears 

 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 

 type of gametophore is the upright stem with the leaves ar- 

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

 some species of Mnium, Hypnum, etc., the gametophore is 

 more or less dorsiventral ; but in these the apical cell is pyram- 

 idal, 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, how- 

 ever, morphologically distinct from the protonema. Thus if 

 ii 



