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4a
Assuming that during half the twenty-four hours the shade-
plants are not able to assimilate to any appreciable extent, Oxalis
Acetosella, for example, will lose every 24 hours about 12 x 0.
3.6 mg. CO, per 50 cm.’ at 18°. In order that a real balance may
be reached this loss must be made up during the day, which, with
a CO, percentage of 0.5: mg. per litre and at 18", takes place
at an average lightintensity of 4, and this therefore denoles the
absolute point of equilibrium.
If to this be added the loss in respiration through non-assimilating
parts (root-system, leaf-stalks, etc.) and the necessary increase of the
dry weight for growth and propagation, it will be seen that the shade-
plants must in reality have access to not inconsiderably greater quan-
tities of light than 45, and hence it is easy to understand the impor-
tance of the patches of direct sunlight in the forest (cp. p. 58).
Assuming for the sun-illuminated patches an average intensity
of about }, which should probably agree pretty nearly with the
reality, the intensity of assimilation for Stellaria with a supply
of CO, of 0.s3 mg. per litre of air proved to be about 1.s mg., and for
Oxalis about 2.6 mg., per 50 cm.” for 1 hour at 18°C. From these
figures it can be calculated how great is the increase per cent in the
dry weight of the leaf, when it is illuminated by direct sunlight.
In an Oxalis plant with a leaf-surface of 50 square centimetres
I found the dry weight to be 252 mg. (including leaf-stalks and
rhizomes). If the length of the period of assimilation is taken to
be 44 months, there will therefore be required for the building
up of this plant an excess of about 2 mg. dry substance per day.
During 1 hour’s illumination with direct sunlight the increase in
weight, calculated as carbohydrate, reaches 1.s mg. The respira-
tion of the non-assimilating parts should also be taken into con-
sideration. We therefore find that little more than one hour’s
direct illumination of an Oxalis plant is sufficient to maintain the
most necessary growth, even if the light-intensity during the other
hours of the day is only 4 (cp. above).
In Melandrium the dry weight of the leaves per 50 cm.” surface
is about 1.s times greater than in Oxadis. The need of light must
therefore be greater.
The nearer to the soil the forest-plants grow, the higher will
be the CO, concentration at their disposal. Oxalis, which is one
of the most pronounced of shade-plants, has all its leaves at but
a little distance from the ground which it covers like a carpet (fig. 7).
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