xxiii] ZONATION 287 



careous and the peaty. Certain water plants are decidedly 

 calcophil; Stratiotes aloides^ is one of these cases, while another 

 is Scirpus lacustris'^^ which has been recorded as absent or rare 

 in the Vosges, while it becomes common when the streams from 

 this mountain region reach the Loess alluvium. When the 

 substratum is peaty, on the other hand, the humous acids break 

 up the calcium carbonate, thus rendering the water untenable 

 for lime-loving plants but favourable for others, which are able 

 to live in a solution poor in mineral salts, such as Lobelia, 

 Limrella and Isoetes^, Those plants which can tolerate peaty 

 water, enjoy the great advantage of freedom from the ravages 

 of Water-snails^. 



Lists have been drawn up of the hydrophytes frequenting 

 stagnant and slowly flowing waters in this country, showing 

 that a different assemblage of plants is characteristic of each of 

 these habitats^. This difference is probably due primarily to 

 variations in the aeration. In extremely stagnant waters, which 

 contain much decaying organic matter and are poorly aerated, 

 the higher plants rarely appear. The Lemnaceae, however, form 

 an exception to this rule, since they not only tolerate, but 

 actually require, certain soluble products of organic decom- 

 position. It has been shown that normal growth and multi- 

 plication cannot be sustained in Lemna minor for any length 

 of time in the absence of certain organic, growth-promoting 

 substances, or auximones^. 



A subject on which great stress is laid in descriptive eco- 

 logical studies, is the "zonation" of the hydrophytes which 

 characterises very many water areas. As a typical example we 

 may refer to Magnin's '^ description of the Jura Lakes, where the 

 plants are distributed with great regularity. Passing inwards 

 from the shore, the following order is generally observed. There 

 is, firstly, a littoral zone of plants standing out of the water 



1 Davie, R. C. (1913). 2 Kirschleger, F. (1857). 



3 West, G. ( 1 905), ( 1 908), and ( 1 9 1 o). * West, G. ( 1 908). 



^ Tansley, A. G. (191 1). 

 Bottomley, W. B. (1917); see also p. 81. '^ Magnin, A. (1893). 



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