110 



PLANT SOCIOLOGY 



20 m. to a depth of 35 m. The individual associations were very 

 sharply delimited. 



Linsbauer (1905), Klugh (1925), and Oberdorfer (1928) have con- 

 structed special apparatus for measuring light under water. Linsbauer 

 found in clear mountain lakes at a depth of 1 m. 19 per cent of the total 

 light above water; at 5 m., only 1.4 per cent. 



Plant communities of snow-land require very small amounts of 

 light, but on exposed snowless ridges and in equally bare rock clefts the 

 Potentillion caulescentis, Androsacion multiflorae, etc., have a very 



30 



'00 0,5 1 15 2 2.5 3 3J5m 



Fig. 61. — Light intensity at various depths of water and the distribution of associa- 

 tion in the Esthwaite, England. A, Litorelletum; B, Myriophyllum aUcmifolium associ- 

 ation; C, Isoetetum; D, Nitella flexilis association. (After Pcarsall.) 



high light requirement. According to the investigations of Riibel 

 (1912) at the Bernina Hospice (2,300 m.), the old shrunken snow cover 

 lets very little light pass through. Loose, powdery, fresh-fallen snow, 

 however, allows at least 1/2,000 of the total light to penetrate to a depth 

 of 50 to 80 cm. Communities hke the Salicetum herbaceae or the 

 Polytrichetum sexangularis, which are not exposed until July, have 

 therefore a very small annual light requirement. When in spite of 

 these facts green foliage is found under a meter of permanent snowbank 

 icf. Braun-Blanquet, 1913), the question arises whether this may not be 

 due to a prolonged storage of chlorophyll. 



3. ATMOSPHERIC CARBON DIOXIDE 



Under natural conditions plants obtain carbon dioxide from both 

 the soil and the atmosphere. As carbon is one of the chief constitu- 

 ents of all organic substances, its absorption from the air plays a 

 notable role in the gas exchanges between plants and the atmosphere. 

 The assimilation of carbon dioxide by the forest is very considerable 

 and is subject to decided variations. The ordinary amount of atmos- 

 pheric carbon dioxide averages 0.03 per cent, but under the cover of 

 trees the lower layers of the air may reach a concentration of 0.08 per 



