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DE
supply of CO,. With a lightintensity of j!, to 3, the culminating
point is already reached by the latter, while in the case of Naslur
tium the curve still tends slightly upwards at } light. In both
cases the assimilation is obviously almost directly proportional to
the intensity of light with intensities under j,—;j,. While, then,
the curve of the shade-plants — in good agreement with Brack-
MAN’S theory — passes over into a more or less horizontal line,
indicative of the limiting effect of the carbon dioxide, the curve of
the sun-plants has a gentler course, most nearly related to that of
a logarithmic curve.
The curve for a green alga recently obtained by O. WARBURG
(1919, p. 253) seems once more to point lo a bimolecular reaction.
WanBURG considers it reasonable to assume that the velocity of
the assimilation is proportional to the CO, concentration and to
ihe concentration in another substance with which the carbon
dioxide reacts. It remains to be seen whether this explanation can
lay claim to be of universal application. In higher plants more
especially, a complicating factor is introduced in the diffusion of
the carbon dioxide from the outer air through the stomata, inter-
cellular system, and cell walls, and it is obvious that the assimila-
tion can under no circumstances proceed more rapidly than is
permitted by the resistance of diffusion. O. WARBURG (1919, p.
254) has pointed out the possibility that in regard to curves of
the BLACKMAN type (see BLACKMAN and SMITH 1911) it might be
the resistance of diffusion, and not, as Brackwaw thinks, the small
CO, concentration, that acted as the sharply limiting factor. In
the shade-plants — as I hope to show in another place — the
proportion. between the absorbing surface and the mass of the
chlorophyll bodies is lower than in the sun-plants, which undoubt-
edly points to the correctness of O. WARBURG’S explanation.
Ecologically the shade plants are suited to the low light-intensily
of the forest, and the curves also show that in lights under „, to
oj they work as economically as the sun-plant Nasturtium. But
even in the darkened forest the light varies very strongly, and may
in the sunniest places reach values of j to 3. And since every
spot of ground, except to the north of the trees, is lit up by direct
sunlight several times during the course of the day in the passage
of the sunbeams through the leaves, the shade-plants can only
ulilise the favourable moments if the CO, concentralion is higher
than 0.5: per litre. As will be shown in the next section, il actually
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