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70
it. Nevertheless even here it is probably the topmost leaves that
take over the greater part of the assimilative work, for the light
which has already passed through a leaf is deprived of praclically
all its assimilating power (see WILLSTÄTTER and STOLL 1918, p.
127). The lower leaves are therefore thrown back upon the
quantilies of light that filter between the leaves above them.
Accordingly it is probably to be assumed that in shade-plants the
really assimilating surface is seldom grealer than the surface of
the ground beneath tbe plant.
The comparisons made above between the carbon dioxide pro-
duced by the ground and that consumed again by the covering
of vegelalion ought therefore to apply quile generally for more
thickly-leaved woods. And if it is remembered that it is only
during the lighter hours of the day that the assimilation reaches
the same values as the CO, production, whereas the latter process
goes on day and night with about the same inlensily, we come
to the conclusion that the total CO, production from the ground
considerably exceeds the consumption of CO, in the assimilatory
work of the herbaceous vegelation. "This agrees well with the fact
that wooded ground gels most of ils carbon not from rolted remains
of the undergrowth but from the fallen leaves of the trees. Even
if all the carbon does not pass back in the form of carbon dioxide
of decomposition, but part of it slays in the ground, the foliage
of the trees is nevertheless so extensive in relation to the surface
of the ground that there must be an excess over the undergrowth.
All the carbon dioxide liberated from the ground will naturally
not be available for the vegetation, on account of diffusion, and
above all the wind. But that the carbon dioxide of the ground
none the less considerably contributes to raise the CO, percentage
in the lower strata of the air in the forest will appear from the
following analyses of air, which I carried out both with my own
method and with the aspirator method (10 litres of air in the
course of 2—3 hours) See the following page.
The figures show that the CO, percentage of the air in the
forest varies very greally, but is as a rule considerably higher than
the CO, percentage in the air over the open field. The average
for the latter during August was 0.612 mg. per litre (p. 52). The
air of the forest at the level of the undergrowth shows during the
same period a minimum value of 0627 mg. and a maximum of
1.254 mg. per litre, which is more than twice the normal. As an
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