60 THE SCOTTISH BOTANICAL REVIEW 



unfavourable to most plant species and are now almost 

 unable to live elsewhere where competition is involved. 



2. Certain species of lichens confined to limestone surfaces. 

 A number of these lichens eat deeply into the surface of the 

 rock, which indicates that their metabolic processes are 

 closely connected with their choice of habitat. In their 

 relations to lime they are probably to be classed with the 

 preceding group. Lichens usually avoid surfaces whose 

 nature or exposure subjects them to exfoliation or destruction, 

 and the colonisation of soluble limestone surfaces by lichens 

 may require the presence of special adaptations to prevent 

 extermination. Some of these lichens " sow their seed, " as it 

 were, in holes in the rock, and the thallus is crustaceous, 

 gelatinous, or evanescent, rarely, if ever, foliaceous or 

 fruticose. 



3. Certain aquatics, semi-aquatics, reedswamp, fen, and 

 marsh plants, which flourish where abundance of lime and 

 other salts are supplied by the waters, owing to the physio- 

 graphic relations of the habitat to the river system. 



The abundance or poverty of lime and alkaline salts has 

 been generally considered to be an important differentiating 

 factor between fenland and acid-moorland, and this is a 

 direct result of the different physiographic relations of their 

 water-supply to the drainage system (3) (26). Fen associa- 

 tions chiefly consist of species from reedswamp and marsh. 

 The rapid growth of fen vegetation indicates considerable 

 quantities of easily available nitrogen compounds, since in 

 the majority of the plants there is no reason for thinking 

 they can obtain their nitrogen by heterotrophic means. The 

 nitrogen is therefore probably obtained autotrophically, and 

 may be directly derived from ammonium salts or nitrates 

 correlated with the abundance of lime and other salts in 

 the waters. 



Graebner has pointed out the rapid exhaustion of mineral 

 nutriment that takes place in the waters of moorland pools 

 and small lakes, and how the pools then fill with floating 

 forms of Sphagnum which gradually supplies sufficient humus 

 for other moorland plants to take root and spread upon the 

 surface (2). This poverty of inorganic, nutrient salts in 

 waters immediately derived from acid-moorland surroundings 



