DEVELOPMENT. 21 



by pressing out these contents between two glass slips. The 

 contents should be examined in water, and, if desired as a 

 permanent record, may be mounted in glycerine or one of 

 the compounds of that agent. 



The columella (fig. 9 c), or central mass, hangs, as it were, 

 from the lid of the capsule, and is held in position by the 

 chains of alga-like cells, which are attached to it in all parts. 

 This columella is formed of large, pale, parenchymatous, thin- 

 walled cells ; originally it fills the whole centre of the cap- 

 sule, but is afterwards divided from the cell walls by the 

 differentiation of certain of the cells to form the mother cells 

 of the spores and by the air cavity. After the spore mother- 

 cells are formed, the columella usually perishes or shrivels 

 up, the whole cavity of the capsule being filled with the 

 spores. But in some cases it is persistent, as in the sub- 

 genus Schistidium and in Pottia, where it remains attached 

 to and falls away with the lid (fig. 4, 2 a), and in the genus 

 Polytrichum, where, whilst the lower portion perishes, the 

 upper portion still remains, forming the beautiful diaphragm 

 which closes the mouth of the capsule in this genus (fig. 15, 



34 



Examining the section still further, we notice that at its 

 apex is a dome-like series of thickened cells; this is the 

 operculum (fig. 9 d} as seen in section. Immediately at the 

 base of the operculum, and, as it were, separating it from 

 the mouth of the capsule, is a row of peculiar cells, forming 

 the annulus (fig. go) ; but these cells are only distinctly seen 

 when the section is very thin, and with the higher magnify- 

 ing powers of the ^ or J-inch objective. The cells forming 

 the annulus are very elastic when mature, and by their 

 expansion throw off the operculum. The annulus is some- 

 times formed of a single, sometimes of a double row of 

 cells, and is sometimes absent, as in Tortula anguiculata^ 

 its presence or absence often forming an important aid to 

 the determination of nearly allied species. Proceeding from 

 the top of the air cavity, and inclosed by the operculum, 

 are the layers of cells forming the peristome (fig. 9 p\ the 

 outer peristome proceeding and originating from the lining 

 membrane of the capsule, and the inner one from the outer 



