BRYOPHYTA I99 



playing remarkable uniformity with respect to basic morphological fea- 

 tures. The Bryales are by far the largest group of bryophytes, number- 

 ing about 13,500 species included in 80 families. Although world-wide 

 in distribution, they are particularly abundant in moist northern regions. 

 They grow on rocks, tree trunks, fallen logs, and on the ground, often 

 forming extensive mats. Some grow in dry situations, while a few are 

 aquatic. Some of the largest genera are Fissidens, Lencobryum, Barbula, 

 Tortula, Grimmia, Funaria, Bryum, Mnium, Bartramia, Hypnum, Polyt- 

 richum, and Pogonatum. 



Gametophyte. In nearly all the Bryales the spore produces a proto- 

 nema~a green, branching, septate filament (Fig. 162). Some of the 



Fig. 162. Moss protonema arising from a spore and bearing a bud from wliich an erect 

 leafy shoot will arise, X 100. 



branches penetrate the soil, turn brown, and become rhizoids. The pro- 

 tonema, which is the morphological equivalent of the thallus of Sphag- 

 num, gives rise to an erect leafy stem, the gametophore. This arises as a 

 bud on the protonema and, in most genera, grows by means of a tetrahe- 

 dral apical cell. After formation of the leafy shoot or, more commonly, 

 of several or many leafy shoots, the protonema usually disappears; but it 

 may persist, turn browai, and contribute to the mass of rhizoids that arise 

 from the lower end of the stem. Branching of the stem, where it occurs, 

 is nearly always monopodial (with a true main axis). Generally the 

 leaves are spirally arranged and borne in three vertical rows (Fig. 163). 

 Usually they consist of a single layer of cells, except for a slightly thick- 

 ened midrib, which is nearly always present. The stems of such mosses 

 as Mnium and Polytrichum contain a strand of elongated thick-walled 

 cells, but the stem tissue of most mosses is uniformly thin-w^alled. 



Vegetative propagation in the Bryales is highly developed and varied. 

 It may occur by isolation of branches following death of the older parts 

 of the plant, by small multicellular gemmae, by resting buds (bulbils) on 

 the protonema, or by the formation of a protonema from almost any part 

 of the plant. 



