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82
Even at raised CO, tension Oxalis maintains its advantage. With
a CO, percentage in the forest of 0.ss mg. per litre and 1 light,
Oxalis assimilales about 27 % more intensively than Slellaria nemorum.
With weaker light (j;) the assimilation in the air of the forest seems
to be about the same for the different species.
The circumstance mentioned may perhaps help to explain the
extreme shyness of Oxalis in regard to the light. Of the shade-
plants examined Ozalis is cerlainly the one that can live under the
weakest light. Since the greater assimilating.power of this plant
chiefly makes itself felt in stronger light, there seems to lie herein
some evidence for the theory advanced by me that the flashes of
direct sunlight falling on the ground are of dominating importance
for the life of the under-vegetation.
In the sun-plants the strong rise in temperature induced by the
constantly active sunlight involves several disadvantages, the chief
of which is the greatly increased transpiration. Often the sun- plants
lose in the middle of the day so much water that the stomala are
closed, and the assimilation is thus greatly impeded or ceases alto-
gether (see THopay 1910, pp. 443 ff.).
If the temperature rises above the optimum point, which, espe-
cially in the case of low-growing species, may easily occur, the
velocily of assimilation is diminished. Another circumstance to be
noted in regard to the sun-plants is the accumulation of assimi-
lation-producis, which with continuously favourable conditions of
assimilation may reach such a degree as to arrest the assimilating
process. It may therefore happen that the daily production of carbo-
hydrates, calculated on the basis of short assimilation-experiments,
turns out to be higher than is indicated by the estimalions of dry
weight (see LuBIMENKO 1908). x
In the forest the light is the factor which most sharply limits
the assimilalion; in the case of the sun-plants it is the carbon
dioxide. The roselte plants and the creeping herbs, however, are
probably relatively favoured, since, coming as they do into imme-
diate contact with the ground, ihey are able to utilise the carbon
dioxide produced by it (cp. p. 68). The nature of the soil here
plays a great part, and the luxuriant vegetation of soil which con-
tains an abundance of organic subslances (e. g. manure) is certainly
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