AQUATIC AND TERRESTRIAL PLANTS 97 



lives submerged in the water, which envelops them completely or, at 

 most, leaves definite floating parts of them uncovered at its surface : 

 these are water-plants (aquatic). 1 On the other hand there is a still 

 greater number of plants that expose at least their assimilating organs 

 to the air and hence to transpiration : these are land-plants (terrestrial), 

 and amongst them we include marsh-plants. 



To submerged water-plants transpiration is an impossibility ; in 

 land-plants transpiration takes place, and it is incumbent upon them 

 to maintain a balance between the intake and output of water, that is 

 to say, they must regulate transpiration. If evaporation be greater than 

 the supply of water the plant withers, and this has the gravest effect 

 upon vital processes, even when it does not go so far as to kill the plant. 



Here reference may also be made to the part played by water in the 

 general economy of nature, in its promotion of putrefaction and of the pro- 

 duction of humus, as the micro-organisms responsible for these processes 

 need water. The significance of water in relation to the distribution 

 of plants is demonstrated most distinctly in flat countries such as the 

 western parts of Denmark ; a marked zonal arrangement of the vegeta- 

 tion reveals itself here, not only in water but also on land, round every 

 lake or pool. Differences of a few centimetres in the level of the water- 

 table suffice to evoke wide distinctions in the vegetation. 2 



The story of man also indicates the importance of water to the plant. 

 History has shown to what an extent the prosperity of countries (density 

 and wealth of the population) is dependent upon water. In Asia, for 

 example, civilization was confined to those lands where a well-watered 

 soil ensured the existence of man. In Algiers the density of population 

 runs almost parallel with the amount of rainfall. 3 Lack of water is that 

 factor in plant-life in the face of which man is most helpless. 



The effect of their environment on water-plants is not only marked 

 by the absence of transpiration, but is also impressed upon them by the 

 other peculiar conditions belonging to water, such as its absorption and 

 consequent weakening of light, its dissolved air, its movements, its buoy- 

 ancy, and other characters. 



In the succeeding chapters the general structural relationships of the 

 water-plant and land-plant respectively will be considered. 



CHAPTER XXVIII. ADAPTATIONS OF WATER-PLANTS 

 (HYDROPHYTES) 



VARIOUS structural features and phenomena exhibited by submerged 

 parts are to be regarded as adaptations to the peculiar physical qualities 

 of water. Some of these are dealt with in the succeeding paragraphs. 



i. Roots and analogous organs. Since nutriment may be absorbed 

 by the whole permeable surface of all submerged parts, there is in sub- 

 merged plants a reduction in those organs which normally extract mineral 

 food-material from the soil, that is to say, in the roots, or in analogous 



1 Hydrophyta of Schouw, 1822. 



* See Raunkiar, 1889; Wanning, 1907; Massart, 1893, 1908, and others.' 



1 Deherain, 1892. 



WARMING H 



