TIIK HRYOl'IIYTA 319 



structure is more highly differentiated than in any other of the 

 bryophytes. A core of sterile tissue, the columella, occupies 

 the center or axis. Around this is a layer of spores, which in 

 many species is broken up by groups of sterile cells. The wall 

 outside this is three or four cell layers in thickness, and except 

 for the outermost one, or epidermis, its cells are provided with 

 chlorophyll and are often separated somewhat by intercellular 

 air spaces. Well developed stomata, with guard cells much like 

 those in the higher plants, occur in the epidermis. The sporo- 

 phyte of the Anthocerotales is therefore able to carry on photo- 

 synthesis actively, though it still necessarily depends upon the 

 gametophyte for water and mineral salts. This group has always 

 been of particular interest to botanists as suggesting a possible 

 connection between the bryophytes and those higher plants in 

 which the sporophyte is an independent individual. 



Musci or Mosses. — The mosses are much commoner and more 

 familiar plants than the liverworts, and under certain conditions 

 form an important element in the vegetation. Many of them 

 thrive only in moist situations but others are common on ordinary 

 dry soil and still others live under exceptionally xerophytic 

 conditions where few plants can grow. The moss-plant is 

 typically erect and consists of a stalk around which small, delicate 

 leaves are arranged in spirals. The stem has very little internal 

 differentiation and the leaves are only one or two cells in thick- 

 ness, so that the vegetative organs are far from approaching in 

 complexity those of ferns and seed plants. As in the liverworts, 

 the plant is attached to the soil by thread-like rhizoids. The 

 sporophyte also shows an advance over earlier conditions in a 

 progressive increase in sterile tissue, particularly in the higher 

 forms; and in opening by a distinct lid, or operculum, at the top. 



Two main orders are recognized, the Sphognales and the 

 Bryales. 



1. Sphagnales or Peat Mosses. — These all belong to the single 

 large genus Sphagnum, characteristic of the bogs and swampy 

 regions of temperate climates. The spore germinates into a 

 flat, thallus-likc structure from the surface of which arise upright 

 and much-branched shoots, thickly covered with small leaves 

 (Fig, 193). Many of the leaf cells are dead and empty and are 

 so constructed that they will absorb and hold large quantities of 

 water. At the tips of the main branches are borne the sexual 

 organs. The globular capsule is provided with a well-devclopiMl 



