482 



VENTILATING SYSTEM 



retain air in their cavities, when the rest of the velamen becomes 

 tilled with water. The inner or narrower edge of each wedge-shaped 

 mass is attached to a single, more rarely to two or three, thin-walled 

 air-containing exodermal cells (Fig. 191 l), which in their turn abut 

 against one or more rows of special rounded parenchymatous elements 

 (Fig. 191 l) ; these last-mentioned cells were termed " cellules aquifdres " 

 by Janczewski on account of the colourless and watery nature of their 



-n> 



Fig. 191. 



Pneumathode of a photosynthetic root of Taeniophyliam Zollingeri (T.S.) ; w, 

 velamen ; e, exodermis ; I, exodermal cell containing air, its lower right-hand wall 

 perforated by a large hole ; r, green cells of the cortex ; /, complementary cells. 



contents, but should more properly be compared to the complementary 

 cells of lenticels, in view of the fact that they are separated from one 

 another by wide intercellular spaces which extend as far as the aforesaid 

 thin-walled air-containing cells of the exodermis. In Tacniojjhyll/ni) 

 Zollingeri, the author has noticed that the inner wall of this cell breaks 

 down in one or two places where it abuts against an air-space ; in this 

 way large holes arise which permit of free communication between the 

 cortical air-spaces and the cavities of the thin-walled cells of the 

 exodermis. 



Since the outer walls of the air-containing exodermal elements 

 always remain intact, the pneumathodes of these Orchidaceous roots 

 do not actually represent .open outlets of the ventilating system ; but 

 Schimper has shown that all the cell-membranes in such pneumathodes 

 are highly pervious to air, so that a sufficiently active gaseous inter- 

 change can go on by diffusion through the cell-walls. 



