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65
From these figures it appears that in Oxalis, with the light at ,
the assimilation is increased in direct proportion to the supply of
CO,. The limit is reached at about 2.0 mg. per litre, i. e. more than
three times the CO, percentage (see fig. 5). For Melandrium also.
under the same conditions, the curve at first takes a rectilinear
course (see table VIII). Here however the limit is alreadv reached
at twice the CO, percentage (about 1.2 mg.; see table VIII). In
Stellaria the assimilation at 4, is increased much more slowly in
respect to the CO, supply: the limit is reached at six times the
amount of CO,.
At about 1 light none of the shade-plants examined shows direct
proportionality between assimilation and CO, supply. The increase
in assimilation proceeds at a slower rate than the CO, supply
would suggest. We may therefore say that the advantage of a CO,
pressure above the normal appears most strongly at low intensities
of light.
Of the sun-plants, Viola tricolor and Nasturtium palustre, as is
shown by fig. 4, seem to use up the CO, supply entirely at !
light. Brown and EscoMBE (1902) give for Helianthus figures which
point in the same direction. In two parallel experiments in »strongly
diffused light» (probably 1—1) the quantities of CO, per litre (rec-
koned in volume) were respectively 2.s and 25.5s per 10,000. The
ratio between them is 1:6... The ratio between the inlensilies of
assimilation was 1:7.», and therefore about the same. In another
case with still stronger light the ratios 1: 4.1 and 1 : 5.s respectively
were obtained. The only statement made about the light in ex-
periments III and IV of the same investigators is that it was »in-
sufficient», and the authors do not seem to have been able to come
to any certain conclusions regarding the importance of the supply
of carbon dioxide at low light-intensities.
A priori it can hardly be doubted that exceptions to the rule of
proportionality occur in the sun-plants also. I think I have myself
found a case of this kind, in Afriplex latifolium. The light-curve
for normal CO, percentage shows at.1:2 light an assimilation of
about 7.5 mg. With a CO, supply of about 3Xx0.5; 13.2 mg were
assimilated, with a CO, supply of about 5x 0.5, 16.» mg. Here
therefore, as in the shade-plants at } light, the assimilation is in-
creased much more slowly than the CO, supply.
5. — Svensk Botanisk Tidskrift 1921.
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