— 
pe 
influence on the vegetation in the winter. It is much 
more probable that the controlling factors in this season 
are the varying effects of light and temperature. From 
the foregoing it is clear that many factors play a part in 
controlling the algal population of the shore. It would 
appear that rising temperature and brighter light at first 
encourage algal growth, provided that adequate nutrient 
salts and dissolved gases are available, but the limits of 
variation in an upward direction for both temperature and 
insolation aresoonreached. Thisis proved by the fact that 
in more southerly latitudes the grand period of algal 
growth falls earlier and earlier in the year, until in the 
Mediterranean region optimum conditions for algal growth 
occur in the winter and early spring, while summer 
forms a “ dead season.” 
Periodic observations have shown that during the 
autumn, individuals of certain species with a wide bathy- 
metric range tend to disappear from the upper levels of 
their area of distribution, but may be found in quantity 
in lower levels where they were formerly not so prevalent. 
This downward migration may be continued until in the 
height of the unfavourable season individuals of the 
species in question may still be found in sheltered places 
in the low-lying levels of the shore but are absent, at all 
events in macroscopic form, from the middle and upper 
zones. Dredging or examination of the “ cast-ups”’ 
after a sudden storm will reveal the presence of these deep 
water survivors. 
The explanation of this 
‘ ’ 
“migration ’’’ may lie in the 
fact that physical conditions of upper pools, offering 
as they do a greater amplitude of variation when compared. 
with the pools at lower levels, automatically become 
unsuitable habitats for the development of a given 
species. Released spores will therefore either fail to 
germinate or will grow only into a microscopic winter form 
' 
