492
MARY BELLE ALLEN
Since plants are immobile, they are especially inclined to temperature changes in sunlight. Photosynthesis utilizes only a portion of the
energy absorbed by a green leaf; the remainder goes to raise its temperature. In shade, on the other hand, evaporation of water may cool
leaves below the temperature of their surroundings. Roots generally
follow the temperature of the soil around them. Succulents, because of
their large ratio of volume to surface, are particularly prone to being
warmed above the temperature of their surroudings by sunlight. Excess
temperatures of as much as 30° have been reported (23, 24).
Internal heat due to metabolism, although generally less than that
produced by homoiothermic organisms, is also of importance to poikilotherms. The ratio of surface to volume is of considerable importance in
determining the contribution which metabolic energy makes to the
thermal economy of the organism. Large creatures, e.g. tuna fish and
pythons, can reach temperatures as much as 10° above their surroundings as the result of active metabolism (25, 26, 27), whereas smaller
forms rapidly lose their metabolic heat through exchange with the
environment. However, insects which are individually too small to retain
much metabolic heat may do so by congregating in swarms (28). Intensive muscular activity may also result in raising even small organisms,
flying insects for example, above the temperature of their surroundings
(15, 29). Among plants, rapidly metabolizing tissues such as palm shoots,
lily bulbs, and parts of cycads have been observed to reach temperatures 10-20° above that of the surrounding environment (30, 31, 32).
Organisms lose thermal energy to the environment by a number of
processes. These include: (a) conduction, which is of greater importance to aquatic forms than to terrestrial ones, since water is a much
better conductor of heat than is air; (b) convection, which is involved
in processes such as transport of heat to the body surface by circulating blood, and loss of thermal energy from the surface to moving
air; (c) radiation, which for a black body is given by the equation
S = 5.75 X 10"
9
T
4 mw./cm.
2
Since the radiation emitted by bodies in the biological temperature
range is of very long wavelength (^ 10 μ), the visible color of an
organism will have little effect on its ability to radiate (12).
Evaporation of water plays an important role. The extent of its contribution depends on the relative humidity of the surrounding environment and on whether the air is still or moving. In dry air, insects
(33-35) and plants (30, 36) have been observed to maintain temperatures materially below those of the surrounding atmosphere as a result
of evaporative cooling. Thin skinned amphibians rely to a great extent
MARY BELLE ALLEN
Since plants are immobile, they are especially inclined to temperature changes in sunlight. Photosynthesis utilizes only a portion of the
energy absorbed by a green leaf; the remainder goes to raise its temperature. In shade, on the other hand, evaporation of water may cool
leaves below the temperature of their surroundings. Roots generally
follow the temperature of the soil around them. Succulents, because of
their large ratio of volume to surface, are particularly prone to being
warmed above the temperature of their surroudings by sunlight. Excess
temperatures of as much as 30° have been reported (23, 24).
Internal heat due to metabolism, although generally less than that
produced by homoiothermic organisms, is also of importance to poikilotherms. The ratio of surface to volume is of considerable importance in
determining the contribution which metabolic energy makes to the
thermal economy of the organism. Large creatures, e.g. tuna fish and
pythons, can reach temperatures as much as 10° above their surroundings as the result of active metabolism (25, 26, 27), whereas smaller
forms rapidly lose their metabolic heat through exchange with the
environment. However, insects which are individually too small to retain
much metabolic heat may do so by congregating in swarms (28). Intensive muscular activity may also result in raising even small organisms,
flying insects for example, above the temperature of their surroundings
(15, 29). Among plants, rapidly metabolizing tissues such as palm shoots,
lily bulbs, and parts of cycads have been observed to reach temperatures 10-20° above that of the surrounding environment (30, 31, 32).
Organisms lose thermal energy to the environment by a number of
processes. These include: (a) conduction, which is of greater importance to aquatic forms than to terrestrial ones, since water is a much
better conductor of heat than is air; (b) convection, which is involved
in processes such as transport of heat to the body surface by circulating blood, and loss of thermal energy from the surface to moving
air; (c) radiation, which for a black body is given by the equation
S = 5.75 X 10"
9
T
4 mw./cm.
2
Since the radiation emitted by bodies in the biological temperature
range is of very long wavelength (^ 10 μ), the visible color of an
organism will have little effect on its ability to radiate (12).
Evaporation of water plays an important role. The extent of its contribution depends on the relative humidity of the surrounding environment and on whether the air is still or moving. In dry air, insects
(33-35) and plants (30, 36) have been observed to maintain temperatures materially below those of the surrounding atmosphere as a result
of evaporative cooling. Thin skinned amphibians rely to a great extent