xn] WATER PORES AND AIR PASSAGES 167 



(4) The stomates may develop normally, but the guard cells 

 remain pressed together 

 with their cuticular ridges 

 interlocked, e.g. Calla pa- 

 lustris (Fig. 107 A). 



In addition to ordinary 

 stomates, which, in sub- 



'. '. FIG. 107. A. Calla palustns, L. T.S. stomate 



merged life, are incapable in submerged leaf stalk; the thickening bands 

 of pvprrkino- their normal fit closely together. B. Potamogeton natans , 



L. T.S. submerged stomate from leaf stalk 



function, Submerged leaves of floating leaf. This stomate is entirely 

 ! i i roofed in with cuticle. [Porsch, O. (1905).] 



also very commonly bear 



water stomates, which are probably of importance in keeping 



up the 'transpiration' stream by exudation 1 . A longitudinal 



section passing through the water pores of Pistia Stratiotes is 



shown in Fig. 53, p. 82, while the apical 



opening of Potamogeton densus in which 



the tracheids communicate directly with 



the exterior without the intervention of 



water stomates is represented in Fig. 



108. 



The aerating system of submerged 

 leaves is a very conspicuous feature. 

 The mesophyll of such subcylindrical 

 radical leaves as those of Littorella and 

 Lobelia Dortmanna is traversed from end 



to end by air passages, interrupted only ^ iog Pota ** geton 

 by porous diaphragms, and the same densus, L. L.s. apex of 

 feature is markedly developed in the medi^ner^e InT showing 

 elongated petioles of such leaves as Sagit- the apical opening. (Upper 



. ,T^. * N <-ni j- i surface of leaf to right 



tana (Fig. 8, p. 19) Inese diaphragms hand .) fx 220< ) [Sauva- 

 form points d'appui for the secondary 8 eau > c - N*9**M 

 nerves connecting the longitudinal bundles 2 . 



The mesophyll of submerged leaves shows, as has been 

 already indicated, little sign of differentiation into palisade and 



1 This subject is considered more fully in Chapter xxi. 



2 Duval-Jouve, J. (1872). 



