18 
to a maximum in May and early June, though fertile 
individuals may be found much later. August marks 
the maximum reproductive period for Pelvetia canaliculata 
whose antheridial conceptacles may be so numerous 
as to paint an orange stripe visible some distance away, 
round the cliffs at high water mark. Fucus serratus 
prefers the autumn for reproduction and plants with 
plentiful receptacles may be found throughout the 
winter. 
Clearly there must be for each genus in its appropriate — 
season some co-operative influence of all the effective 
factors of the environment, stimulating the plant to 
copious reproduction. For Fucoids and Laminarians and 
some other genera, reproductive structures are confined 
to special limited portions of the thallus: in other cases 
reproductive organs are produced all over the plant 
surface. The initiation and nourishment of these cells 
must tax the resources of the plant to the uttermost so 
that when large numbers of them filled with food material 
have been liberated, there is left behind a depleted thallus 
whose surface, ruptured to allow the escape of generative 
cells, now offers itself a prey to colonisation by the spores 
of epiphytic genera. When reproductive cells are liberated 
in successive crops extending over a long period of time, it 
is obvious that the expenditure of energy involved and the 
amount of accumulated material dissipated must eventually 
bring about complete depletion of the plant’s vitality 
and lower its resistance to the forces tending to destroy it. 
Of the latter, the most effective is undoubtedly the 
strain of moving water, and when, as frequently happens, 
periods of rough weather coincide with the period of 
maximum reproductive activity, the result is the tearing 
of thalli and their removal on a wholesale scale. 
The fact that depopulation is quickly made good by 
an upgrowth of sporelings or by proliferation from basal 
