462 



VENTILATING SYSTEM 



cells. Each stoma thus opens into a cavity, the atmosphere of which 

 remains undisturbed, and hence highly charged with water-vapour, 

 however the surface of the leaf may be swept by air-currents. Pfitzer 

 was the first who correctly interpreted the "sunken" stoma as an 

 arrangement for preventing excessive transpiration. It w r as subse- 

 quently shown by Tschirch, that the various forms of stomata found in 

 members of the xerophilous Mediterranean and Australian floras mostly 

 conform to this general type of structure. Many epiphytes have 

 stomata of a similar character, and an external air-chamber may even 

 be developed in a moist tropical climate, in the case of plants which 

 are liable to transpire very actively in bright sunlight, owing to the 

 fact that their leaves are both numerous and large {e.g. Ficus elastica). 



A. Stoma of Cypripedium venustum (upper side of leaf) (T.S.). Wide front cavity (v) 

 overarched by the large cuticular ridges of the guard-cells. v400. B. Stoma of 

 Dasylirion fili.folium (T.S.). External air-chamber divided into two compartments 

 (a and a J by projecting ridges. xtiTO. 



The hyperstomatic chambers containing stagnant air, which are of 

 such general occurrence among xerophytes, do not all originate in the 

 same way. In Cypripedium, venustum, Olivia nobilis, etc., these chambers 

 consist of the enlarged front-cavities of the stomata (Fig. 176 a), while 

 each of the very slightly sunken stomata on the photosynthetic stems 

 of Euphorbia Tirucalli is surrounded by a thick annular wall of wax 

 (Fig. 177). According to Volkens, each of the stomata which occur 

 on the outer surface of the leaf of Aristida pungens is overarched by 

 four solid cuticular pegs ; these meet over the stoma in such a w r ay as 

 to form a vaulted chamber, which only communicates with the outer 

 atmosphere through exceedingly narrow crevices. 



As a rule, each stoma has its own external chamber; certain plants, 

 however (Nerium Oleander, spp. of Goscinium), develop larger and 

 deeper chambers, with numerous stomata on their walls, which may be 

 termed collective or common external air-chambers. In many steppe- 

 and desert-plants, in the genus Casuarina, etc., these common air- 

 chambers are replaced by longitudinal furrows bearing stomata on 

 their sides. 



