AQUATIC TYPE OF STOMA 



467 



"terrestrial" type; as a rule, closure of the stoma, instead of being 

 brought about by apposition of the bulging ventral walls of the 

 guard-cells, is entirely dependent upon the more or less close approxi- 

 mation of their external cuticular ridges, which are always well 



a 



A 



Fig. 182. 



Unequal development of stomata on the two sides of a leaf. A. Stoma from the 

 upper leaf -surf ace of Populas pyramidalis ; guard-cells somewhat depressed, so that 

 a shallow external air-chamber results (). A l Stoma from the lower leaf-surface; 

 depression of guard-cells barely perceptible. B. Stoma from the upper leaf-surface 

 of Plantaao major ; B\ stoma from the lower leaf -surface. The former has the wider 

 front cavity (v). (All in T.S.) 



developed (Fig. 183). The differentiation of front cavity, pore passage' 

 and back cavity is incomplete or altogether absent; the pore expands 

 rapidly beneath the narrow fissure enclosed by the cuticular ridges, and 

 thus opens directly by a wide aperture into the hypostomatic air- 



Fig. 1S3. 



Stomata of the aquatic type. A. Trianea bogotenxis. B. Zemna minor. 

 C. Alsophila sp. (All iii T.S.) 



chamber. Stomata of this type occur, for example, in Lemna minor, 

 Trianea oogotensis, Hydrocharis Morsus Banae, Nywphaca alba, Victoria 

 regia, Euryale ferox, and Trapa nutans. In certain other cases, on the 

 other hand, in which the cuticular ridges are equally well developed 



