Chap. I] 



WATER 



25 



in members (rhizomes, roots) from which such canals are constantly absent 

 in terrestrial plants, is to be ascribed to the risk of want of oxygen owing 

 to the slow diffusion of gas in water ; these canals conduct the oxygen set 

 free by assimilation into tissues that are not green. Plants growing in 

 water that is in active movement and consequently richly supplied with air, 



Fig. 31. Ranunculus fluitans. Transverse section of a segment ot a leaf, a Aquatic form; 

 magnified 90. b Terrestrial form ; magnified 60. After H. Schenck. 



such as the Podostemaceae of tropical waterfalls and the larger surf-Algae, 

 are distinguished from terrestrial plants neither by a large development of 

 the surface, nor by the possession of special aerating devices. These 

 phenomena will be more thoroughly discussed further on l . 



The other characteristics of aquatic plants may be considered as due to 

 the direct action of the water. Three of them are characteristic of liquid 

 water, as opposed to water- 

 vapour : first, the poverty in, 

 or absence of, stomata which 

 no longer function as in air 

 as organs for the interchange 

 of gases, since the whole 

 surface of an aquatic plant 

 absorbs and emits oxygen 

 and carbon dioxide and no 

 transpiration takes place ; 

 secondly, the central position 

 of the vascular bundles in 

 correspondence with the 



necessity for resisting tensile strains ; finally, the mucilage, which protects 

 young plants against loss of substance by diffusion 2 . Other peculiarities 

 also occur in a less pronounced degree in damp air, namely, the reduction 

 of the root-system, of the vessels, and of the epidermal tissues, and this 

 reduction is correlated with the absence of transpiration in water and with 

 its diminution in damp air : to this may be added the slight development 



Fig. 32. Callitriche stagnalis. Transverse section of 

 stem, a Terrestrial form, b Aquatic form. Magnified 

 67. After H. Schenck. 



1 See Part I, Chap. IV, and Part III, Sect. V. 



Schilling, op. cit. 



