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76
be the case. A wet summer is as a rule followed by an early
autumn (from the point of view of the forest vegetation). On the
other hand the production of CO, is more abundant during a wet
summer. Compensation for the smaller amount of light is afforded
in thick and dark woods by the there prevailing maximum CO,
concentration in the air.
To make an estimate of the daily production of carbohydrates
in the shade-plants proves very difficult, on account of the strong
variations in the light-intensity and available supply of carbon
dioxide. A calculation can only be based upon continuous obser- |
vations of the light and the CO, concentration in the habitats of
the plants, and these observations I have not yet been able to
make. I shall therefore at present limit myself to an approximate
estimate of the necessary minimum conditions.
The estimations given in the present paper concern the relative
intensity of assimilation, which is somewhat lower than the actual
intensity, on account of the continuous respiration. Ecologically
however it is just this relative intensity that is of interest. The
point where respiration and assimilation counterbalance each other
is the point where the curve cuts the abscissa. With normal CO,
percentage this point of equilibrium lies at light-intensity ;3, to
zip’: With higher light-intensity the assimilation gives a positive |
result, with lower light-intensity the respiration dominates and the
leafs funds of carbohydrates are diminished. Owing to the higher
CO, tension prevailing in the forest, the point of equilibrium should
probably be pushed back to a light-intensity of about „15. — This
relatively low intensity however does not denote the actual minimum
of existence. For in the nocturnal respiration carbon dioxide is lost.
The shade-plants examined have the following intensity of respi-
ration, calculated per 50 cm.?, 18°, and 1 hour.
Oxalis Acetosella 0.30 mg. ,
Melandrium rubrum 0.29 »
Stellaria nemorum 0.32 »
* The value obtained by BoysEN-JENSEN (1918, p. 249) for the point of equilibrium |
in Oxalis, viz. 0.2 (Bunsen units X 100), corresponding to ahout ‚1, according to
my scale, seems to me somewhat too low. The value is probably to be explained
by the fact that with BoysEn-JENsEN’s method some CO, is always lost in the
absorption, so that e. g. the respiration values must turn out too low. A per.
fectly exact calculation of the position of the point of equilibrium is naturally
rendered more difficult by the heterogeneity of the material, whatever be the
method employed.
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