464 



VENTILATING SYSTEM 



lined by mechanical cells or cell -processes of peculiar shape. In the 

 former case each air-chamber is shut off from the palisade-tissue by an 

 irregularly twisted, knobby stereide. The lateral processes of this 

 " protective cell " are all firmly attached to the neighbouring cells of 

 the fibrous hypodermis. It is evident that the air-chamber is not 



Fig. 178. 

 Stoma from a cladorle of Ruscus aculeatus (T.S.). After H. v. Guttenberg. 



hermetically sealed against the adjoining tissues. As Tschirch says, 

 such a chamber may be compared to a cave with its mouth closed by a 

 rock of very irregular outline; communication with the interior of the 

 leaf is restricted, but not absolutely prevented; gases can pass freely 



between the protuberances of the protective cell. 

 In Xantliorrhoea a similar partial occlusion of the 

 internal air-chamber is effected with the aid of 

 inward prolongations of the adjoining elements 

 of the sclerenchymatous hypodermis (Fig. 180 a). 

 According to Pfitzer and Gilg, very remark- 

 able arrangements for the restriction of trans- 

 piration are found, in connection with the internal 

 air-chambers, in the Eestiaceae, a xerophilous 

 family of Monocotyledons peculiar to South Africa 

 and the Australian Continent. In most cases 

 (e.g. Elegia nuda, Restio tectorum, R. panieulatus, 

 Dovea mucronata) each of the air-chambers, which 

 project deeply into the photosynthetic tissue, is 

 surrounded by a group of columnar " protective stereides," with 

 cutinised walls.. These stereides form an uninterrupted sheath around 

 the upper or stomatic end of the chamber, but separate slightly from 

 one another below ; the narrow intercellular clefts produced in this 

 way might almost be compared to a second, internal system of stomata 

 (Fig. 180 b). In Restio nitens and Lepidobolas Preissianus the internal 

 air-chamber is enclosed, except at its lower end, by a funnel-shaped 



Fig. 179. 



Stoma from the lower side 

 of the leaf of Nipa fruticans 

 (T.S.) After Bobisut. 



