294 



MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



dry, and even if that particular part of the plant is wholly immersed, they do not 

 come into contact with the water. There are two causes for the unevenness of the 

 leaves: first, the outer walls of a portion of the superficial cells may become strongly 

 arched outwards; or secondly, solid peg-like projections may arise from the cuticle, 

 and to these projections the air adheres so firmly that it cannot be displaced even 

 by a considerable pressure of water. This protection of stomata against moisture 

 by papilla-like outgrowths is to be found especially in marsh plants which are 

 exposed to a changing water-level. On the banks of streams and rivers, and 



Fig. 64.— Stomata. 



1 Surface view of a portion of the frond of the fern Nephro&ium Filix-mas. 2 Vertical section through this portion, 

 s Surface view of a portion of the leaf of Peperomia arifolia. * Vertical section through this portion ; x 350. 



where water welling up from below forms pools and ponds, it may happen that 

 plants are submerged for a week at a time, and then again remain dry for some 

 months. 



Most of the plants growing in such situations, particularly the sedges (e.g. 

 Carex striata and paludosa), the rushes (e.g. Scirpus lacustris), most of the tall 

 fistular grasses (Glyceria spectabilis, Phalaris arundinacea, Eulalia japonica), the 

 plants which grow with the sedges (e.g. Lysimachia thyrsiflora, Polygonum 

 amphibium), and many other marsh plants, are all saved from the danger of 

 having their stomata wetted during their submersion by the papilla-like out- 

 growths of some of the epidermal cells, near the stomata, as shown in the figures 

 on next page. 



Bamboos, and the grasses Arundmaria glaucescens and Phyllostachys bam- 



