138 



MOSSES AND FERNS 



CHAP. 



of the sporogonium is occupied by a columella composed of 

 sixteen rows of cells, which in cross-section form a nearly per- 

 fect square. At the base these cells are thin-walled and show 

 no intercellular spaces, but farther up their walls begin to 

 thicken and the rows gradually separate until in the upper part 

 the columella has somewhat the appearance of a bundle of 

 isolated fibres. The archesporium is constantly growing from 

 below, and the new cells are cut off from those surrounding the 

 columella in the same way as at first. 

 The archesporium, as well as the colu- 

 mella, can be traced down nearly to the 

 base of the capsule, and its cells are very 

 early recognisable both by their position 

 and by their contents. At first but one 

 cell thick, the archesporium soon be- 

 comes double, but does not advance be- 

 yond this condition. As the archespo- 

 rium is followed from the base towards 

 the apex of the capsule the cells begin 

 to show a differentiation. Up to the 

 point where the archesporium becomes 

 divided into two layers the cells appear 

 alike; but shortly after this their walls 

 begin to separate, and two distinct 

 forms are recognisable, arranged with 

 much regularity in many cases, although 

 this arrangement is not invariable. 

 Pretty regularly alternating are groups 

 of oval, swollen cells, with large nuclei 

 and abundant granular cytoplasm, and 

 much more slender ones, that may un- 

 dergo secondary longitudinal divisions. 

 The latter have smaller nuclei and more 

 transparent contents. Examination 

 higher up shows that the former are 

 the spore mother cells, the others the elaters, which here have 

 the character of groups of cells, and do not develop the spiral 

 thickenings found in most Hepaticae. As these two sorts of 

 cells grow older they separate completely, and the spore mother 

 cells become perfectly globular. The sterile cells remain more 



Fig. 71. — Anthoceros Pear- 

 soni. Median longitudinal 

 section through the base 

 of the sporogonium. The 

 archesporium is shaded. 

 F, Foot; V, V, basal 

 sheath of calyptra, X 100. 



