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53
With a view to shortening the time of absorption as much as pos-
sible I used 7, solutions. 3 litres of normal air (= the capacity of
the apparatus) contain about 1.:;—1.s mg. CO,, which corresponds
to about 1.55 ccm. of 3, oxalic acid. With an accuracy in reading
to 0.0 ccm., an error of about 1.5 4 therefore arises. The estima-
tions of air are therefore certain at about "us = (0.43 %, as far as the
error in reading is concerned. In the assimilation however all the
amount of CO, should not be used, but only a fraction of it. In
experiments with ordinary air I allowed the CO, percentage to be
reduced by + to + on the average, which corresponds to about 0.5 ccm.
of oxalic acid. The error will therefore be 4%. With raised CO,
percentage and increased assimilation the error is of course relatively
less. In a further paper I shall describe the results of a number
of test-experiments with known amounts of CO,.
With accurate work, therefore, the constant error should be quite
inconsiderable in comparison with the numerous accidental sources
of fallacy which are involved in the genetic and modificatory varia-
lions of the material, and which must always be taken into account
in ecological work. It is therefore necessary to make many esti-
mations with constantly fresh material, and this, thanks to the sim-
plicity of the method, it is easy to do.
In calculating the results one must of course take into considera-
lion the volume of the glass tube gl (fig. 1).
Illumination. As source of light the natural daylight was used.
Some experiments were carried out in the very habitats of the plants:
but since the light in the forest is subject to constant variations,
I carried out the majority of the experiments on the open place
outside the Station.
Close to a wall facing NW, which is in the shade from morning
till about 4 p. m., there could be obtained a strongly diffused light,
the intensity of which during the months of June, July, and August
constituted in clear weather about } of the maximum light of heaven.
I took as unit the maximum light of heaven, i. e. the intensity in
the open field under full sun at noon in the month of July.
The light-measurements were carried out with an Imperial Ex-
posure-meter, No. 1. The experiments during the summer of 1920
were favoured with good weather. Beneath a sky covered with
drifting rainclouds the intensity of light varies too much to be used.
Lower intensities than 1 were obtained by laying photographic plates,
with varying darkening powers, over the apparatuses.
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