20 
mean temperatures for air are less than those for the sea 
during the period from September to March, but exceed 
them during the summer months. There are thus two 
points—early April and late August, when mean tempera- 
tures for sea and air are approximately equal. 
Temperature variation is, however, by no means the 
most potent factor controlling algal activities. It is 
difficult to dissociate the influence of rising temperature 
and of increasing insolation, for as a rule both factors 
show variation in the same direction at the same time. 
In the spring, however, when the rate of algal growth 
undergoes sudden acceleration it is probable that the 
rising value of incident hght involving increased length 
of daylight as well as of greater intensity of illumination, 
acts as the stimulating factor, for a scrutiny of tempera- 
ture records (Fig. 1) shows that no very great rise in 
the temperature of the sea-water takes place until late 
in the spring, long after the growth acceleration has 
shown itself. The somewhat higher air temperatures in 
the late spring do not play any great part in encouraging 
algal activity since the most marked growth appears in 
the lower third of the littoral zone, amongst algae that 
are not exposed for long periods to the air. On the 
other hand, rising air temperatures of high summer 
acting on algae during periods of long exposure or affecting 
the temperature of pools at high levels on the shore, 
may reach a point that exceeds the range of toleration 
of many species inhabiting the littoral zone. Co-operating 
with this rise in temperature are increased exposure to 
illumination, enhanced risk of dessication, and increase 
in the salinity and in the pH of the pools, during the 
periods when exposure occurs in the middle of the day. 
From the data collected one would suspect that tem- 
perature and insolation together are limiting factors in the 
summer time. With these factors must also be considered 
