142 



MOSSES AND FERNS 



CHAP. 



in the epidermis of elongated leaves of Monocotyledons. In 

 the older parts some of these cells cease to elongate, and be- 

 come more nearly oval (Fig. 75, A). These are the young 

 stomata, and exactly as in the vascular plants, each divides 

 longitudinally by a septum which later separates in the middle 

 and forms the pore surrounded by its two guard cells. The 

 walls of the other epidermal cells become much thickened and 

 distinctly striated. Each epidermal cell contains two large 

 chloroplasts like that in the cells of the gametophyte, and be- 

 tween the cells are well-developed air-chambers communicat- 

 ing with the stomata, so that there is here a typical assimilative 

 system of tissues. 



The doubling of the chloroplast in the cells of the sporophyte 

 has been noted by Schimper (A. F. W. Schimper (2)), and 



Fig. 74. — Ripe spores and elaters of A. Pearsoni, X6oo. 



this was observed by the writer in both A. fusiformis and A. 

 Pearsoni. 



About the base of the growing sporogonium is a thick 

 tubular sheath representing in part the calyptra of the other 

 Hepaticae, but involving, besides the archegonium venter, also 

 the surrounding tissue of the gametophyte. This sheath keeps 

 pace with the growth of the sporophyte for a long time, but 

 finally the sporogonium grows more rapidly and projects far 

 beyond it, and this remains as a tube surrounding its base. 

 The growth of the sporogonium continues as long as the 

 gametophyte remains alive, and in A. fusiformis is often 6 



