SUBSIDIARY CELLS 471 



the other hand, and the gametophores in all cases, are provided with 

 tubular air-pores ; these are more or less barrel-shaped structures, 

 consisting of several superimposed tiers of cells, enclosing a tubular 

 passage (Fig. 187). The lowermost tier usually differs from the 

 others, and was regarded even by Mirbel as a closing apparatus, witness 

 his name of anncau obtvrateur. Experimental evidence as to its 



I y 



Fig. 1S7. 



Air-pore of Mareha/ntia polymorpha. Above, as seen in vertical section ; below, as seen 

 from below, in order to sbow the papillose cells of the lowermost tier. 



function has been obtained by Kamerling, who succeeded in demon- 

 strating that these lowermost cells can in many cases effect partial 

 {Fimbriaria Stahliana), or even complete (Plagiochasma Aitoniana), 

 closure of the pore. This basal tier usually consists of relatively large 

 cells, and tends to contract more or less actively when turgor is 

 reduced. The tubular pores of Marchantia polymorpha are immoveable. 

 Here each of the pore-cells of the basal tier is provided with a large 

 inwardly directed papilla-like process ; as a result, the pore only com- 

 municates with the underlying air-chamber through a comparatively 

 narrow cruciform fissure. It is not improbable that this arrangement 

 is designed to retard gaseous interchange, and thus to restrict 

 transpiration. 



6. Subsidiary cells. 



Very frequently, those cells of the epidermis which immediately 

 adjoin the guard-cells of stomata, differ in structure from the typical 

 epidermal elements ; cells of this kind pertain both anatomically and 

 physiologically to the stomatic apparatus in the wide sense [and are 

 hence termed the subsidiary cells of the stomata]. The function of 

 these subsidiary cells is not precisely the same in all cases. In the 

 Cyperaceae and Bkomeliaceae their thin tangential walls act as 

 "hinge-areas." Where an external air-chamber is present, its sides 



