BRYOPHYTA 



193 



tetrahedral apical cell, and from each of its three rows of segments a row 

 of leaves arises, these being spirally arranged. 



The leaves are composed of a single layer of cells and lack a midrib. 

 They are peculiar in structure, some cells being enlarged, rhomboidal, and 

 hyaline, while others are small, narrow, and green (Fig. 158£'). The 

 green cells form the meshes of a network enclosing the hyaline cells. The 

 latter are dead cells filled with water; their walls bear large circular pores 



B 



Fig. 157. Sphagnum. A, young gametophyte, showing filament arising from the spore, 

 a rhizoid, and the thallus beginning to develop by an apical cell; B, mature thallus, with 

 rhizoids, producing a leafy shoot; C, an antheridium arising between two leaves, X250. 

 (A arid B, after Schimper.) 



and usually spiral thickenings as well. The leaves have an extraordinary 

 power of absorbing and retaining water. At first the leaf cells are 

 uniform, but later from each a narrow cell is cut off on two sides, as 

 represented by Fig. 1585-Z). 



The stem of Sphagnum is differentiated into three regions: (1) a cortex 

 of dead hyaline cells that absorb and store water; (2) a cylinder of small 

 elongated cells with thick walls; and (3) a pith-like axis. Vegetative 

 propagation occurs by branching and death of the older parts of the plant. 

 This is the principal method of reproduction. 



Sex Organs. Depending on the species, Sphagnum is either monoe- 

 cious or dioecious. The antheridia appear on special, short, lateral 

 branches that arise near the apex of the main shoot. They are solitary in 

 the leaf axils, unaccompanied by paraphyses, and arise in acropetal sue- 



