470 



VENTILATING SYSTEM 



Azolla and Funaria is quite unknown. It is conceivable that the 

 obliteration of the septa serves to prevent any inequality of burgor- 

 pressure on the two sides of the pore. The condition of Polytrichwm, 

 where the " one-celled " stomata are accompanied both by the four- 

 celled and . by the two-celled forms, seems inexplicable except as an 

 instance of variety of design. 



Among Liverworts, typical two-celled stomata do not occur, except 

 on the sporogonium of Anthoccros. In the March anti ales the gameto- 

 phyte is furnished with characteristic pneumathodes. But these differ 



so markedly from the stomata in other 

 Bryophyta and Vascular Plants, that they 

 are preferably distinguished under the name 

 of air-pores. 230 



According to Leitgeb these air-pores may 

 be either simple or tubular. A simple air- 

 pore typically occupies the centre of an 

 approximately hemispherical raised area of 

 the epidermis, and is surrounded by several 

 concentric series of epidermal cells. In 

 Fegatella conica (Figs. 185 and 186) the 

 walls of the innermost circle of cells are 

 produced towards the circular pore in the 

 form of a membranous border which thins out to a sharp edge, so that a 

 vertical section of the pore recalls the structure of a stoma of the aquatic 

 type. In this Liverwort, all the cells immediately underlying a pore are 

 produced upwards into long, colourless, beak-like processes of unknown 



Fio. 18 



Air-pore of Fegatella conica. 

 (Surface view.) 



Fig. 186. 

 Aif-pore of Fegatella conica. (Vortical Section.) 



significance. Kamerling's suggestion that these cells represent an 

 " evaporating apparatus " is purely speculative. Simple air-pores of 

 various kinds are characteristic of the vegetative branches of the 

 majority of Marchantiales. The thalli of Prcissia and Marchantia, on 



