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81
1908, WILLSTÄTTER and STOLL 1918, p. 118), the shade-leaves are
able to absorb and utilise the greatest possible proportion of the
light available.
The relatively low and constant ground-lemperature in the forest
should also help to bring it about that the sunbeams which pene-
trate in patches do not succeed in causing too great heating, with
consequent lessening of the relative dampness of the air. The strong
light of the sun-illuminaled patches is utilised in the assimilation,
since the stomata are not closed, as I have ascertained by infiltration
experiments.
In the case of Oxalis the stomata are opened even in the direct
sunlight, and return to their half-closed condition in the diffuse
light. On a sunny patch under alders and ferns a light-intensity
of jl; was measured (Oct. 4), and in another place the intensity
was i—1. The stomata, except in older leaves, were completely
open. In shady patches they were half- or three parts closed. In
a bed of Oxalis at the foot of a powerful alder the leaves on the
sunny side showed immediate infiltration with xylol. On the shady
side the infiltration proceeded much more slowly, or did not take
place at all.
The opening and closing of Oxalis stomata takes a time of j—1
hour, and they therefore remain open for a while after the sunny
patch has passed by. Only in very strong and continuous sun-
illumination, in more open places in the forest, does the opposite
reaction apply, when the stomata close again. This is probably
due to the losses by transpiration, since the leaves hang down and
in other respects show signs of relaxing. — In Viola palustris also
I have observed fully open stomata in the sun-illuminated patches.
Here however the stomata were fully open even in the shade.
Fully open stomata of course ha e a highly favourable effect
upon the supply of CO, to the assimilating cells, and from the
observations given the conclusion may be drawn that the shade-
plants fully utilise the opportunilies for increased assimilation offered
by the combination of direct sunlight and high CO, concentration.
The temporarily increased temperature of the leaf is also a not
unimportant factor here.
From the curves in fig. 2 and from the table on p 56 it will be
seen that the assimilating-power of Oxalis is about 50 % greater than
in the other shade-plants with normal CO, pressure and strong
light, i. e. when the carbon dioxide acts as the limiting factor.
6. — Svensk Botanisk Tidskrift 1921.
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