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58
happens that the air in the forest often contains more Carbon
dioxide than in the open field. The curves in fig. 2 therefore give
the lower limit. They indicate that under the CO, conditions of
the sun-plants the shade-plants are decidedly inferior to these.
In order to gain information about the conditions of light in the
habitats I have made a number of measurements. The plants
experimented upon cover in magnificent profusion the drier portions
of two fair-sized alder-bogs on Hallands Väderö — »Kapellhamn
Bog» and »Ulagap Bog». In the absence of direct sunlight very
low light-intensilies prevail here. In August, at 9,30 a. m., the
light on a Polystichum-Circea-Oxalis-association under a slightly
clouded sky was found to be only j;. Specimens of Oxalis under
ferns had only 41, light. As soon as the sun broke out, inten-
sities of + could be measured in the directly illuminated patches’.
In an alder-wood, and still more in a beech-wood, however, the
sun-patches are small and therefore soon slip to one side.
In a Stellaria nemorum-Oxalis-association under a deeply shading
beech-tree the following intensities were measured in succession
from 2 to 3 p. m. on a sunny day at the end of June: — 4c, 4,
zs, 35. By 6,30 p. m. the intensity had fallen to 317. Here there-
fore the carbohydrate production of the plants should probably be
referred to the few hours in the middle of the day, when they are
under stronger light. In a lighter part of the wood, under oaks,
the following intensities were measured in sunny weather (July 7,
12 noon —3 p. m): — 5; 4, yy, ty, 4 1, 4, 4, & The vegetation
here consists of Rubus idaeus.
It is generally known that the light in the forest is more in-
lense in spring, before the leaves are fully out, and in autumn,
when they are falling (see HEssELMAN 1904, WIESNER 1907). For the
plants here examined however — with the exception of Oxalis —
this fact is of no importance, since they run through practically
their whole development during the summer. — On Oct. 3 I
measured the lightintensity in sunny weather at 12 o’clock noon
in »Ulagap Bog» and found in one of the darker places 4;—,
in a lighter place j;. The maximum light of heaven has at this
lime of the year gone down to 3.
The above figures may certainly give an approximale idea of
’ Since, according to Dorno (1919), the direct sunlight has a relatively weaker
effect upon photographic paper than the diffuse light of heaven, these values are
probably too low.
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