35 
plants do not grow at the surface but appear as epiphytes 
on other algae growing on the sides of the pool at a depth 
of about a foot. One gains the impression that the 
factors controlling conditions at the shallow end of the 
pool are no longer favourable to the growth of Asperococcus 
fistulosus. Still later in the year, during the summer 
months, the plants disappear from the median position 
and are only to be found near the bottom of the pool 
at the deep end, where illumination is diminished and 
where the temperature is less subject to daily rise by 
insolation. This localised movement of Asperococcus 
within the confines of one pool is paralleled by the 
movements of the plants on the open shore. In the 
winter Asperococcus fistulosus is to be found only as 
meagre individuals in shallow well-lit pools of the upper 
zone. It relinquishes this position in the spring and 
appears as a component of the flora of pools much lower 
down the shore where the shorter period of daily exposure 
counteracts the increasingly strong spring sunshine. 
The downward migration is continued until Asperococcus 
is to be found as an inhabitant of pools lying at the level 
of ordinary neap tides. 
The factors determining these migrations are difficult of 
analysis. It would appear that a falling value for light 
intensity is not the imperative factor for plants that 
move downwards during the winter since a move in such 
direction leads to further diminution of the already 
reduced autumn and winter daylight. Neither is scarcity 
of mineral salts likely to cause migration since the work of 
Harvey, Atkins, and others has shown that the nitrate 
and phosphate content of sea-water though depleted in 
the autumn is replenished in the winter. The factor that 
suggests itself as being directly concerned with these 
migrations is that of changing temperatures. In the 
summer the air temperature rises above that of the sea, 
