CLIMATIC AND EDAPHIC FACTORS 



403 



A great many of the special structural features of stomata are 

 designed to impede the passage of gases and thus to hinder the escape 

 of water-vapour through the internal and external air-chambers, or 

 through the pore itself. All such arrangements act in the same 

 general way, namely, by constricting or complicating the path followed 

 by the diffusing gases. In a very large number of cases, the external 

 air-chamber is partially occluded by wax, in the form of numerous 

 granules or of a more compact porous plug. This particular arrange- 

 ment was first observed in certain Conifers by Link and later by Wil- 

 hehn, while Wulff has recorded its occurrence in a number of Angio- 

 spermous families (Gramineae, Liliaceae, Betulaceae, Casuarineae, 

 Sileneae, etc.). The external air-chambers of Dasi/lirion filiforme are 



Fig. 177. 



Stoma of Euphorbia Tirucalli (T.S.). External air-chamber surrounded by an annular 



wall of wax. 



constricted in a totally different manner; here a pair of stout flat- 

 topped ridges project inwards from opposite sides of the chamber, 

 which is thus divided into two superimposed compartments, approxi- 

 mately equal in size and communicating with one another by means 

 of an exceedingly narrow slit (Fig. 176 b). According tp Guttenberg, 

 a very similar state of things is found in Ruscus aculeatus, where the 

 front cavity of the stoma is divided into upper and lower halves by a 

 pair of (sharp-edged) cuticular ridges, w r hich only leave a very narrow 

 fissure open (Fig. 178). 



We are indebted to Bobisut for a description of the very peculiar 

 stomata of Nipa fruticans. In this case the ventral wall of each 

 guard-cell is furnished with several cuticular ridges of various sizes 

 and shapes; the ridges of the two guard-cells fit into one another 

 in the most exact manner, as a glance at the adjoining illustration 

 (Fig. 179) will show. 



Very frequently it is the internal air-chamber that is provided with 

 arrangements which tend to restrict gaseous interchange. In Kinr/ia 

 anstralis and Xanthorrhoea hastilis, according to Tschirch, the chamber is 



