A ^ B 



Fig. 107. A. Callapalustyis,'L. T.S. stomate 

 in submerged leafstalk; the thickening bands 

 lit closely together. B. Potamogeton natans, 

 L. T.S. submerged stomate from leaf stalk 

 of floating leaf. This stomate is entirely- 

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



xii] WATER PORES AND AIR PASSAGES 167 



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

 remain pressed together 



with their cuticular ridges 7P=v;\^ //^ ^^>^^/^^")/' 



interlocked, e.g. Calla pa- 

 lustris (Fig. 107 /), 



In addition to ordinary 

 stomates, which, in sub- 

 merged life, are incapable 

 of exercising their normal 

 function, submerged leaves 

 also very commonly bear 

 water stomates, which are probably of importance in keeping 

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

 section passing through the water pores of Pistia Stratiotes is 

 shown in Fig. ^1^^ p. 82, while the apical 

 opening oi 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 

 by porous diaphragms, and the same 

 feature is markedly developed in the 

 elongated petioles of such leaves as Sagit- 

 taria (Fig. 8, p. 19) These diaphragms 

 form points d'appui for the secondary 

 nerves connecting the longitudinal bundles^. 



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 [)uyal-Jouve, J. (1872). 



C.S. 



Fig. 108. Potamogeton 

 densus, L. L.S. apex of 

 leaf passing through the 

 median nerve and showing 

 the apical opening. (Upper 

 surface of leaf to right 

 hand.) (x 220.) [Sauva- 

 geau, C. (1891I).] 



