490
MARY BELLE ALLEN
However, for many qualitative and semiquantitative purposes, particularly when it is desired to compare one organism or reacting system
with another over a limited temperature range, a simple linear temperature coefficient suffices. This is usually expressed as Q 10 , the increase in
reaction rate caused by a 10° rise in temperature. When a wide temperature interval is investigated, Q 10 usually varies with the temperature, since the actual temperature-rate relationship is logarithmic rather
than linear.
II. Methods of Meeting Thermal Requirements
A. POIKILOTHERMS
Although all organisms generate heat in the course of their metabolism, the lower organisms do not have a mechanism for using this
metabolic energy to maintain a constant internal thermic environment.
Although larger forms may become temporarily warmer than their surroundings during periods of activity, small organisms have too great a
heat loss to maintain a temperature appreciably different from that of
their surroundings (with certain exceptions, which are discussed below). In all cases, however, these poikilothermic organisms are dependent on an environment which will provide a temperature favorable
for their life processes. Under natural conditions, this usually means
that their metabolism must cope with a great deal of diurnal and seasonal temperature variation. This is sometimes reflected by alterations
in the optimum temperature and temperature coefficients of enzyme activity. The biochemical basis of these alterations is little understood.
Some organisms, notably the higher plants, have even become dependent on regular temperature differences between day and night for
the proper completion of their development cycle (10, 11). Here again,
the biochemical reactions underlying this need for a temperature cycle
are practically unknown. (Discussion of the temperature cycles of
higher plants is considered to be outside the scope of this review. For
exposition of this subject see Went, 11).
The primary external source of thermal energy for most organisms
is sunlight. (The exceptions are warm springs and other environments
which contain an alternative source of heat.) The total solar radiation
reaching the earth is approximately 2 cal./minute/cm.
2 (12). The fate
of this solar energy is shown diagrammatically in Fig. 1. For organisms
at the surface of the soil both direct and reflected sunlight contribute
to the thermal energy available.
The extent to which this radiation raises the temperature of an organism depends upon its thermal conductivity. Thin leaves rapidly
MARY BELLE ALLEN
However, for many qualitative and semiquantitative purposes, particularly when it is desired to compare one organism or reacting system
with another over a limited temperature range, a simple linear temperature coefficient suffices. This is usually expressed as Q 10 , the increase in
reaction rate caused by a 10° rise in temperature. When a wide temperature interval is investigated, Q 10 usually varies with the temperature, since the actual temperature-rate relationship is logarithmic rather
than linear.
II. Methods of Meeting Thermal Requirements
A. POIKILOTHERMS
Although all organisms generate heat in the course of their metabolism, the lower organisms do not have a mechanism for using this
metabolic energy to maintain a constant internal thermic environment.
Although larger forms may become temporarily warmer than their surroundings during periods of activity, small organisms have too great a
heat loss to maintain a temperature appreciably different from that of
their surroundings (with certain exceptions, which are discussed below). In all cases, however, these poikilothermic organisms are dependent on an environment which will provide a temperature favorable
for their life processes. Under natural conditions, this usually means
that their metabolism must cope with a great deal of diurnal and seasonal temperature variation. This is sometimes reflected by alterations
in the optimum temperature and temperature coefficients of enzyme activity. The biochemical basis of these alterations is little understood.
Some organisms, notably the higher plants, have even become dependent on regular temperature differences between day and night for
the proper completion of their development cycle (10, 11). Here again,
the biochemical reactions underlying this need for a temperature cycle
are practically unknown. (Discussion of the temperature cycles of
higher plants is considered to be outside the scope of this review. For
exposition of this subject see Went, 11).
The primary external source of thermal energy for most organisms
is sunlight. (The exceptions are warm springs and other environments
which contain an alternative source of heat.) The total solar radiation
reaching the earth is approximately 2 cal./minute/cm.
2 (12). The fate
of this solar energy is shown diagrammatically in Fig. 1. For organisms
at the surface of the soil both direct and reflected sunlight contribute
to the thermal energy available.
The extent to which this radiation raises the temperature of an organism depends upon its thermal conductivity. Thin leaves rapidly