468 



VENTILATING SYSTEM 



(Potamogeton natans, Limnanthcmum nymphoides and Alisma nutans), 

 the closure takes place in the ordinary manner by apposition of the 

 ventral walls. 



With regard to the physiological significance of this " aquatic " 

 type of stoma, it is quite certain that the mere absence of any serious 

 danger of excessive transpiration does not suffice to explain the pre- 

 valence of such stomata among floating water-plants. In the author's 

 opinion, the arrangement in question serves to diminish the risk of 

 capillary occlusion of the stomata by water; for liquid cannot lodge 

 between the sharp edges of the cuticular ridges, except in the form of 

 a thin film which readily bursts, and is quickly removed by evapo- 

 ration. 



Stomata of a very similar type have been observed by the author 

 in a large proportion of the plants inhabiting the constantly moist 

 wooded glens near Tjibodas, on the slopes of Gedeh in Western Java; 

 the species in question belong to the most diverse taxonomic groups 

 (e.g. Filicales, Commelynaceae, Acanthaceae, Gesneeaceae). 



5. The stomata of Gh/mnosperms, Pteridophytes and Bryophytes. 



Among the Archegoniatae, deviations from what has come to be 

 regarded as the normal type of stoma are even more frequent than 



they are in the Angiosperms. These 

 modifications are undoubtedly in part 

 adaptive in character, while in other 

 cases they merely represent cases of 

 variation of design. A phylogenetic 

 classification of the various types of 

 stoma which occur in the Arche- 

 goniatae, has been attempted by Porsch. 

 From the researches of Kraus, 

 Tschirch, Mahlert, Klemm, Porsch and 

 others, 22S it appears that the stomata 

 of most Gymnosperms conform to a 

 type of construction which is principally characterised by the fact, that 

 the front cavity is reduced to a narrow fissure, of uniform bore or slightly 

 dilated in the middle (Fig. 184). There are no internal cuticular 

 ridges, and the space enclosed by the ventral walls of the guard- 

 cells passes gradually over into the internal air-chamber, so that there 

 is in such cases nothing comparable to the pore passage and back cavity 

 of the normal stoma. According to Porsch, feebly developed internal 

 ridges are present in a few cases (e.g. Podocarpus neriifolia); here it is 

 permissible to speak of a back cavity which communicates with the 

 hypostomatic chamber through an unusually wide aperture. As a rule, 



Fig. 184. 



Stoma of Juniperus macrocarpa. (T.S.) 

 After Klemm. 



