MUSCI 



which for a time fills the spore-sac, composed of two or three layers of 

 barren cells immediately enclosing the mass of spores. Between the 

 spore-sac and the wall of the mature 

 sporange is an annular air-cavity, tra- 

 versed horizontally by rows of chloro- 

 phyllous cells, the trabecules. The 

 opercule is simply a piece of the epi- 

 derm of the sporange. In most genera 

 a portion of the wall of the sporange, 

 situated near the base of the columel, 

 consists of an assimilating system 

 composed of spongy or palisade- 

 parenchyme, containing chlorophyll, 

 and marked by the presence of sto- 

 mates. If the detachment of the oper- 

 cule leaves the mouth of the sporange 

 with a smooth edge, it is said to \yegym- 

 nostomous, as in Pottia (Ehrh.). More 

 often the mouth of the open sporange 

 is furnished with hair- or tooth-like 

 appendages, arranged in one or two 

 rows, constituting the peris tome. The 

 single row of these appendages, or the 

 outer row if there are two, are called 

 teeth, the inner row cilia. In some 

 genera the cilia are furnished with 

 lateral processes uniting them with one 

 another, or they are replaced by a 

 lattice-work of longitudinal or trans- 

 verse ridges termed the endostome. The 

 inner and outer layers of both teeth 

 and cilia differ from one another in 

 their hygroscopic capacity; hence, as 

 the moisture of the air varies, they 

 bend inwards and outwards, or some- 

 times coil spirally round one another 

 (Barbula, Hedw.). The peristome has a 

 very beautiful appearance under the 

 microscope, and its structure furnishes 

 useful characters for the discrimination 

 of the genera. In most genera the teeth and cilia are not composed of 

 cells, but of pieces of thickened cell-wall which become detached from a 



L 2 



FIG. 112. A, longitudinal section of spo- 

 range of Polytrichum piliferum Schreb. 

 ( x 15). B, transverse section ( x 5 '.. w, wail 

 of sporange ; cu, opercule ; c, columel ; p, 

 peristome ; ep, epiphragm ; a, annulus ; 

 z, air-spaces traversed by trabecules ; s, 

 spore-sac ; ap, apophyse ; st, seta. (After 

 Lantzius-Beninga.) 



