STABLE AND MIGRATORY PLANT FORMATIONS 6l 



should be contrasted with the fenland conditions, where such 

 salts are constantly replenished by the river water, and 

 induce a rapid growth of rooted plants through the greater 

 part of the period of subaqueous humus accumulation. The 

 humus accumulation in fen is primarily of physiographic 

 origin, from the subaqueous nature of the habitat, and from 

 the rapid growth and periodical extensive death that 

 characterises the vegetation. 



Fen-humus is protected from erosion or much admixture 

 with inorganic silts by the reed-belt and the physiographic 

 relations of the habitat. An evolutionary succession in plant 

 association accompanies the change from reedswamp to fen, 

 with the gradual accumulation of humus up to flood water- 

 level. The spongy nature of the humus, and its relation to 

 the water-level, ensure a fairly constant supply of alkaline 

 waters at various depths, and vigorous species of the reed- 

 swamp persist in close association with an increasing number 

 of marshland species, which invade the surface of the humus 

 as the upper limit of the water is approached (3) (25). Fen- 

 land differs markedly from moorland floristically and ecologi- 

 cally, but its relations to reedswamp and marsh are ver}' close. 

 In any defined area, the aquatics, reedswamp, fen, and marsh 

 are related by a similarity in their supply of inorganic food 

 but are segregated by the physiographic conditions which 

 lead to physical differences in the habitat. In comparison 

 with moorland peat the humus substratum of fen appears to 

 be chemically inert, though some of the fen plants are prob- 

 abh' saprophytes. 



The ammonium and potassium salts brought down in the 

 river-waters would be locked up as inert compounds with the 

 humus derivatives of the fenland peat, but where calcium 

 carbonate is present in excess the ammonium and potassium 

 would be set free as carbonates, the calcium taking their place ; 

 or where sufficient inorganic bases were present, the original 

 ammonium and potassium salts would be protected from 

 combining with the humus derivatives (22). Nitrates would 

 probably be more abundant than ammonium salts in the 

 river-waters, but might undergo reduction in the presence 

 of much organic matter, and nitrification of ammonium salts 

 would also be prevented, so it seems likely that fen plants 



