74 



of Ihe coasl, and slill more of an island, ollers more favourable 

 condilions in regard to Ihe supply of CO^ Ihan Ihe air of the 

 counlrv (with Ihe exceplion of mounlain-air). Perhaps Ihis fact, 

 logelher whh Ihe dampness of Ihe air, Ihe equable lemperalure, elc, 

 is a not unimporlanl cause of the grealer luxuriance of ihe vege- 

 tation on islands in the sea. Since the assimilation is in a great 

 measure proporlional lo the CO., pressure, the sniallest constanl 

 difTerences in the CO^ percenlage of the atmosphere must in the 

 long run appreciably alfecl Ihe vegetation. 



Of the absohile quantities of COg introduced inlo the air from 

 the ground, some idea will be given by the following calculation, 

 based npon my vahies. Assuming an average production of 1 mg. 

 j)er öO cm.^ per hour. this makes for one hectare 2 kg. CO,^. In 

 24 hours, 48 kg., in 3 monlhs 4,320 kg. = 2,160,000 litres. Accor- 

 ding lo a calculalion of Ehhhmaykp, (1885) a wood of 1 hectare 

 stores up annually 3,000 kg. of carbon, which corresponds to 11,000 

 kg. of carbon dioxide. Taking the assimilation-period proper as 

 4^ monlhs, there is accordingly produced in 3 months ^. 11,000 

 = 7,300 kg. CO2. Of this quantity, the ground furnishes 4j|g{; = 

 about 60%. bul il is to be noled that part of the carbon dioxide 

 of the ground is formed by Ihe rotting of vegetable remains of 

 undergrowlh. 



(). The assimilation in the forest and on the shore. 



We are now in a position to survey the natural conditions of 

 assimilation under which herbaceous forest- and sun-planls live. 

 The former are subject to a carbon dioxide pressure which varies 

 with different meteorological and edaphic factors, but which during 

 the period of vegetation is always higher than the normal. The 

 curves show that the assimilation then becomes more intensive 

 than wilh normal CO2 supply, and this is so even with low in- 

 tensilies of light. Under favourable condilions the (Ä)._, percenlage 

 in the air of the forest rises up lo twice the amount and more, 

 and the assimilation is then assisled in about the same proportion. 

 Thus the higher CO2 pressure in the forest compensates lo a cer- 

 lain exlent the lower lighl-inlensity there prevailing. 



Tn a Stellaria nemoriim-Oxalis-associaiion in the darker parts of 

 the forest there prevails on sunny days an average light-intensity 

 of ^j; fsee p. 58\ With a CO„ concentration of 0..^,; mg. per litre 



