SECTION VI 



CLASS III. OXYLOPHYTES. FORMATIONS ON 



SOUR (ACID) SOIL 



CHAPTER XLVL XEROMORPHY. FORMATIONS 



On a soil that contains an abundance of free humous acids, and is 

 more or less peat-Hke, there occurs a group of closely-related formations ; 

 these rise in stature from humble communities of mosses and lichens, 

 to dwarf-scrub and bushland, and finally to forest. 



All these communities share the character of choosing soil that is 

 poor in nutriment and particularly in easily assimilable nitrogen they 

 are oligotrophic ; furthermore, they are not calciphilous. They often 

 clothe sterile sand, on which they themselves soon produce raw humus, 

 and usually give rise to dense, exclusive, and extensive communities. 



Sour soil is intimately associated with a moist, cold or temperate, 

 chmate. 



These communities all exhibit xeromorphy, that is to say, they are 

 protected from desiccation by certain devices, of which the most important 

 are : 



1. Well-developed coating of hairs : Hairs on the lower face of the 

 leaf form a felt in Ledum, Salix repens, S. lanata, and S. glauca, but are 

 scale-like in Cassandra calyculata, and in the North American Nyssa 

 uniflora, Persea pubescens, and Magnolia virginiana, which grow in 

 swamps. 1 The essential function of the hairs may be to prevent water 

 from occluding the stomata, but the hairs also depress transpiration. 

 In this connexion it may be noted that Salix Myrsinites, growing in the 

 swamps of Lapland, retains its faded leaves which serve to protect the 

 year's shoot .- 



2. Papillae project over the stomata of various Gramineae and 

 Cyperaceae, such as Carex limosa, C. panicea, and C. rarifiora ; also of 

 Lysimachia thyrsiflora, Polygonum amphibium.^ They also may prevent 

 the stomata from being blocked by water. 



3. Wax forms incrustations over the whole leaf, as in Vaccinium 

 uliginosum, or only over the stomatiferous lower face, as in Andromeda 

 polifoUa, Vaccinium Oxycoccos, Primula farinosa, Sahx groenlandica, 

 Carex panicea, and in the North American swamp-plants,"* Acer rubrum, 

 Persea pubescens, and others. 



4. Thick cuticle is shown by various leaves, and by the stems of Scirpus 

 caespitosus and others. 



' Kearney, 1901 ; see p. 191. ' Kihlman, 1890. 



' Volkens, 1890; Kihlman, 1890; Raunkiar, 1895-9, 1901. 

 * Kearney, 1901. 



WARMING O 



