Su 
The movements of algae wp the shore in the winter are 
probably accounted for by the diminished light of the 
autumn and winter. Many Rhodophyceae are among the 
plants that migrate in this manner and being somewhat 
sensitive to the strength of illumination falling on them 
can only spread upwards during the period of diminished 
insolation. In the spring the rising values of the incident 
light act as a limiting factor to the upward movement 
and the high values of summer insolation may actually 
cause the death of plants in the upper levels of their 
area of distribution. Another factor may also take 
a prominent part in the summer zoning of the algae, 
namely the change in pH of the pools subject to great 
insolation. The pools at the upper limits of the tidal zone 
with an exclusively green algal population frequently 
show pH values as high as 9-6. Such conditions encourage 
the growth of Chlorophyceae but prove unsuitable habitats 
for the growth of Phaeophyceae and still more so for 
Rhodophyceae. 
It has been shown that periodicity in algal activity is 
controlled by the physical factors consequent on life in 
a moving medium and also by physiological changes in 
other factors of the environment. The factor that limits 
the advance of a genus is not necessarily also a limiting 
factor for other genera or even for other species of the 
Same genus. Nor is the proportionate influence of 
component factors the same for all plants; nor is it 
constant for one plant at all times of the year. Whatever 
be the relations between an alga and the component 
factors which together make up its environment, the 
onset of unfavourable conditions is inevitably heralded 
by copious reproduction ; the last act of the threatened 
vegetation is the production of propagative cells, just 
as the formation and maturation of seed marks the 
finale in the annual life-cycle of the land plant. 
