INTERNAL AIR-CHAMBERS 



-I .->.-> 



The back cavity of every ordinary stoma opens into a more or less 

 extensive air-space, which communicates at various points with the 

 ventilating system. The regular occurrence of this internal air-chamber 

 or ventilating chamber is readily understandable, in view of the fact 

 that the stomata are the [principal and sometimes the only] outlets 

 of the ventilating system. Not infrequently, special mechanical pro- 

 vision is made to prevent these air-chambers from collapsing, when 

 the organ in which they occur is subjected to bending stresses. 

 Where the stomata are placed in longitudinal series, cjj. in the 



Fig. 173. 



Mechanical protection of the internal air-chamber. A , A ,. Air-chamber overarched 

 by green mesophyll-cells (T.S.). A. Ehjmus. A v E. arauuius. B, B^. Air-chambers 

 of Iris germanica, surrounded by a horizontal ring of green mesophyll-cells. (Surface 

 view.) 



majority of Grass-leaves, in Conifer-needles, etc. all the air-chambers 

 of a series generally coalesce to form a continuous passage ; the cells 

 that abut against these passages, are usually arranged in such a manner 

 as to form a series of arches roofing in each passage. Most often each 

 stoma is underlain by a pair of long, curved, photosynthetic elements, 

 which converge so as to meet on the inner side of the air-passage 

 (Fig. 173 a, a x ). In Iris germanica, where the stomata are irregularly 

 scattered, each air-chamber is surrounded by a circular group of three 

 or four tangentially- curved photosynthetic cells (Fig. 173 B, B 1 ). 



The mechanical importance of the arrangements which have just 

 been described, is purely local. Westermaier 221 ascribes a somewhat 

 different significance to the rigid " gutters," composed of thick-walled 

 cells, which enclose the hypostomatic air-passages (formed as in the pre- 

 ceding instances by the longitudinal fusion of adjacent air-chambers) in 

 the haulms of Scirpus rarspitosus, Erioplwrum alpinum, etc. (Fig. 171). 

 Here the longitudinal seriation of the stomata weakens the tangential 



