236 XVIII. STRUCTURE OF THE BRYOPHYTA. 



from the walls which bound it laterally. In the midst of the free 

 outer wall is found the air- pore (the so-called stoma), which we 

 now see to be surrounded by from about four to eight tiers of 

 cells (Fig. 93, B). The pore is narrowed at its upper and under 

 apertures, especially at this latter, and therefore appears barrel- 

 shaped. The cells of the uppermost tier are prolonged into a 

 membranous border. As the air is very tenaciously retained in 

 the air-opening, and the structure is thereby made indistinct, it is 

 desirable previously to air-pump the air out of the preparation. 

 Into the air-chamber project from below threads of cells, two or 

 three cells long, and occasionally branched. These threads are 

 especially rich in chlorophyll ; they arise from the flat cell layer 

 next below, which is poor in chlorophyll. On the ventral side 

 of the thallus we see on the midrib the lateral, alternating over- 

 lapping of the median scales. Between the scales lie the cross- 

 sections of the bundles of rhizoids. Median longitudinal sections 

 show the insertion alike of the stronger ordinary rhizoids, turning 

 off at once from the thallus, and the " pegged " rhizoids overlying 

 the midrib. 



