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80
The table shows that the forest has a more even temperature
than the shore, and that the variations in temperature, especially
in the forest near the ground, are small. This agrees with the
experience of other investigators. The thermograph of 1917 gave
the temperature for upper and more exposed parts of the forest,
near the sea. The thermograph of 1918 marked the temperature
in the undergrowth in a shaded place, and probably the values
obtained may be regarded as characteristic of the surface of the
ground. It is therefore these figures that in the present connexion
interest us more closely.
The summer of 1918 was cold and wet, and the low temperatures
near the ground in the forest are therefore not surprising. Maxima
by the shore are evidently to be associated with the heating in-
fluence of the direct rays of sunlight on the ground. That the
minima by the shore do not prove lower than in the forest is
very probably due to the moderating influence of the sea. A
consequence of this is also that the minima do not appear until
between 4 and 5 in the morning. In the more open parts of the
forest, as the estimations for the year 1917 show, the minimum
values are reached one or two hours earlier. Near the sheltered
ground in the forest, on the other hand, minimum values appear
at the same time as on the shore, or even a little later (see the
averages for 1918). It is a general characteristic of the forest that
maxima do not appear until 1—2 hours later in the afternoon
than on the shore.
The conditions of temperature in the forest are therefore evidently
not specially favourable to the balance of carbohydrates. The
relatively high night-temperature favours the respiration, while the
relatively low day-temperature lessens the assimilation. The late
appearance of maximum temperature 2—3 hours after maximum
light is also unfavourable. ;
In other respects, however, the temperature of the forest offers
advantages which indirectly favour the assimilation. The risk of
the overheating of the tissues and of great losses by transpiration -
is excluded. The leaves of the shade-plants are also adapted to
relatively high humidity of the atmosphere and regular conditions
of temperature: they are large and thin, and therefore the chloro-
phyll is exposed to direct illumination in a much higher degree
than is the case with the thick leaves of the sun-plants. Thanks
to this and to the higher percentage of chlorophyll (see LUBIMENKO .
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