OCCLUSION OF INTERNAL AIR-CHAMBER 



465 



sheath composed of inward prolongations of the epidermal elements 



immediately adjoining the subsidiary cells of the stoma. 



Some plants contrive to block up the internal air-chambers of their 

 stomata, either during prolonged periods of drought, or when the guard- 



Fig. 180. 



Partial occlusion or investment of the internal air-chamber by means bf stereides. 

 A. Xanthorrkoea hastilis, after Tschirch ; m, stereides. B. Eleyia deusta. 



cells die or from some other cause lose the power of closing their pores 

 effectually. Most frequently this occlusion depends upon the fact that 

 the immediately adjoining mesophyll cells grow out into the air- 

 chambers after the manner of tyloses. This process has been described 

 by Schwendener in the case of old leaves of Camellia japonica and 

 Prunus Laurocerasus ; the lenticel-like developments observed by the 

 author on the petioles of certain deciduous trees (Acsculus, Acer, Tilia, 

 Juglans) are evidently of a similar nature ; here the air-chambers are 

 filled with tightly packed cells which finally die derived from a 

 special secondary meristem. In Pilea elegans the air-chambers on the 

 adaxial side of the leaf become invaded by parenchymatous cells, con- 

 taining abundant protoplasm but few chloroplasts. These cells show 



2g 



