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74
of the coast, and still more of an island, offers more favourable
conditions in regard to the supply of CO, than the air of the
country (with the exception of mounlain-air). Perhaps this fact,
together with the dampness of Ihe air, Ihe equable temperature, etc.,
is a not unimportant cause of the greater luxuriance of the vege-
tation on islands in the sea. Since the assimilation is in a great
measure proportional to the CO, pressure, the smallest constant
differences in the CO, percentage of the atmosphere must in the
long run appreciably affect the vegetation.
Of the absolute quantities of CO, introduced into the air from
the ground, some idea will be given by the following calculation,
based upon my values. Assuming an average production of 1 mg,
per 50 cm.? per hour, this makes for one hectare 2 kg. CO,. In
24 hours, 48 kg., in 3 months 4,320 kg. = 2,160,000 litres. Accor-
ding to a calculation of EBERMAYER (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
44 months, there is accordingly produced in 3 months 2. 11,000
= 7,300 kg. CO,. Of this quantity, the ground furnishes $320 =
about 60%, but it is to be noted that part of the carbon dioxide
of the ground is formed by the rotting of vegetable remains of
undergrowth.
6. The assimilatien in the forest and on the shore.
We are now in a posilion to survey the natural conditions of
assimilation under which herbaceous forest- and sun-plants 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 CO, supply, and this is so even with low in-
tensilies of light. Under favourable conditions the CO, percentage
in the air of the forest rises up to twice the amount and more,
and the assimilation is then assisted in about the same proportion.
Thus the higher CO, pressure in the forest compensates to a cer-
tain extent the lower light-inlensity there prevailing.
In a Stellaria nemorum-Oxalis-associalion in the darker parts of
the forest there prevails on sunny days an average light-intensity
of jl; (see p. 58). With a CO, concentration of 0.57 mg. per litre
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