SCALE MOSSES 247 



thickened. By the expansive force of these elaters 

 the sporophyte is ruptured somewhat stellately, and the 

 spores are forced out. When the spores germinate they 

 give rise directly to the gametophyte generation. 



433. The Scale mosses (Order JUNGERMANNIALES) 

 are the highest of the Liverworts, and also the most 

 numerous in species. In the lower family (Metz- 

 geriaceae) the gametophyte is usually a thal- 



lus as in the liverworts already described, 



but in the higher family (Jungermanniaceae) 



it is a creeping, leafy stem. In the first 



family we find all gradations from the en- 



tire margined thallus to those with more and- 



more pronounced lateral lobing, and finally 



to those in which the lobes have become distinct leaves 



on a rounded stem. The leaves of Scale mosses are 



but one cell thick and are not ribbed. 



434. The antherids and archegones are" borne dorsally 

 or subterminally and are much like those already 

 described. The sporophyte develops a slender stalk 

 which carries up the enlarged spore case, and the latter 

 when the spores are mature splits vertically into four 

 segments and permits the escape of spores and elaters. 

 When the spores germinate they may develop directly 

 such adult gametophytes as are described above, while 

 in the higher forms the gametophyte is first a filamentous 

 or thalloid structure ("protonema") from which the 

 adult gametophyte subsequently buds out. 



435. Many Scale mosses reproduce by means of brood 

 masses much like those of Marchantia, or even simple, 

 single-celled structures (brood cells). 



436. Scale mosses have no stomata on either gameto- 

 phytes or sporophytes. 



