36 
consequently pools left longest exposed to the air will 
reach a higher temperature than others. Conversely 
the air temperature in the winter falls distinctly below 
that of the sea, so that the upper pools will again 
be the ones to show the greatest range of variation. 
There is therefore a graduated scale of temperature ~ 
observable on any sector of the shore with the highest 
values in pools at the high tide level and a minimum 
value, equivalent to that of the general sea temperature, 
in pools at low water mark. The intervening pools will 
shew intermediate temperatures determined partly by 
their distance from the low water mark and partly by the 
physical proportions of the pools themselves—shallow 
pools being subject to greater temperature rise than deep 
pools during the same period of insolation. Moreover, 
a deep pool may shew well-marked layering of water at 
different temperatures. The upper surface under the 
influence of insolation may rise in temperature but, if no 
mixing of the waters from below occurs, the warmer 
and therefore lighter water will remain floating on top. 
A difference of three degrees between surface water and 
bottom water may be observed in summer in a pool that 
is no more than eighteen inches deep. 
In the winter the temperature gradient may be reversed 
in direction. The lowest temperature will be experienced 
by the algal flora of the upper pools. It follows therefore 
that spring and autumn constitute critical periods 
for the algae, literally turning points in the current of 
invasion. In an interesting way this temperature factor 
serves to discriminate genera and species according to 
their preference for certain conditions and determines the 
direction of their seasonal migrations from level to level 
on the shore. Plants with a preference for warmer 
conditions will move down the shore in the winter and 
re-ascend to higher levels in the following spring. 
