outside > warmer inside) during the hours of darkness, in its func-
tion as depositor or precipitator.
With rising air temperatures, the point where the positive and
negative temperature gradients meet within the tree shifts to deeper
levels. The flow of the sap becomes more sluggish and the positively
charged nutritive elements are held in near stationary suspension at
various heights to await the arrival of the negatively charged ele-
ments from above. This may be the reason why the Amazonian rain-
forest stops producing oxygen towards midday. The positive
nutrient-transporting temperature gradient soon changes to a neg-
ative one as the temperatures rise rapidly during the morning. This
arrests the supply of nutrients; photosynthesis ceases and with it,
the expiration of oxygen is interrupted.
With nightfall and the cooling air, the temperature gradient
reverses to positive from the inside outwards, so that the outer lay-
ers of the tree become cooler than the inner. The crown cools faster,
causing the sap to sink quickly. The higher temperatures of the
Amazon rainforest cause fast evaporation, to bring accelerated
Fig. 15.5. Vertical trunk section.
This shows ring temperature decreasing inwards,
and the flow of nutrients in the xylem, upwards
in daytime, descending at night.- THE METABOLISM OF THE TREE