STABLE AND MIGRATORY PLANT FORMATIONS 59 



These habitats accumulate acid humus, or may fail to do so 

 on account of soil-porosity or the physiographic relations of 

 drainage and exposure. In the latter cases plant-growth is 

 discontinuous and retarded, and the removal of dead 

 vegetation by mechanical or fungal agencies is more than 

 usually effective. 



The calciphobe species thus inhabit sterile soils, where any 

 humus that accumulates is acid in character, inhibiting 

 bacterial action, and the plants have therefore to be of such a 

 nature that the nitrogen, necessary for their growth and 

 metabolism, can be obtained from minute quantities of 

 inorganic nitrogen-salts or from organic compounds, either 

 directly or by heterotrophic, fungal, or parasitic association 



(24) (13). 



Under the calciphobe associations we would therefore 

 include on the one hand those of the heathland, etc., where 

 the surface has been long subjected to leaching ; and on the 

 other hand, those of the moorland and others, where stagnation 

 of the soil-waters leads toacidity and acid-humus accumulation. 

 In the former case lime is removed from the soil by leaching ; 

 in the latter, lime may in some cases accumulate, but is locked 

 up in the form of humates faster than the supply is capable 

 of neutralising acidity. It is probable that variations in the 

 ratio between the rate of development of acidity, and that of 

 neutralisation by calcium carbonate or other bases, lead to the 

 differentiation in the moorland of such associations as grass- 

 moor, calcareous bog, and various t\'pes of moorland "flushes," 

 and also between sour marsh and marsh-meadow, all of which 

 depend greatly on physiographic factors for the differentiation 

 of their habitats. 



The calciphile plants, from the point of view of their 

 physiographic relations, may be roughly sub-divided as 

 follows : — 



I. Aquatics or semi-aquatics, usually found in waters 

 holding excessive quantities of lime in solution, and including 

 a number of algae and some mosses that become encrusted 

 with calcium carbonate, and probably abstract carbonic acid 

 from bicarbonate of lime in solution. These should perhaps 

 be classed with the sinter-organisms, saline plants, and others 

 that have evolved special protective powers against conditions 



