550 , Lloyd Williams . — Studies in the Dictyotaceae. 
not so prejudicial to the plant as is that of epiphytes. Spores of small 
Algae are apt to be caught by the projecting edges of the sori, and if 
their growth is very rapid they deprive the surface cells of much of 
the light they would otherwise obtain. At Plymouth a small green 
Alga particularly affected the antheridial sori, the surfaces of which it 
covered with branching, creeping filaments before they managed to liberate 
their contents. When the hair clusters are eaten off the thallus by marine 
animals, as they frequently are, the access of foreign spores is greatly 
facilitated. When the plant becomes infested in this way with epiphytes 
the result is immediately observable in a diminished production of game- 
tangia, a greater inequality in the ages of the sori, and some amount 
of delay in their maturation. In this connexion the general ecology 
of the habitats becomes important. In some localities most of the 
Algae reproduce in winter or spring, while in others the majority are 
summer- or autumn-fruiting plants. It is clear that Dictyota in the 
former habitats will be freer of epiphytes than in the latter, and their 
own reproduction will be correspondingly more successful. 
During two visits paid to Meadfoot (Torquay) this August, most 
of the sexual sori were found to be several stages later than the Plymouth 
plants. The plants were apparently free from epiphytic growth, but on 
examining them microscopically they were found to be completely covered 
with exceedingly fine, nearly colourless, filaments, seemingly of a cyano- 
phyceous nature, and it was probably this growth that caused the marked 
retardation of the crop. 
It has been proved above that changes in the meteorological conditions 
do not determine the main facts of periodicity for any locality, but that 
they may, and frequently do, bring about slight modifications in the 
time of development by either accelerating or retarding it. One striking 
result of continuously favourable weather conditions is to make the 
development of the sori more uniform. In the diagrams this is visible 
in the narrowness of the crop-bands. When, on the other hand, 
unfavourable weather prevails, the sori are less uniform and the crop- 
band becomes wider. 
If meteorological observations were kept, in order to test the effect 
of weather on the crops, mere daily records of the total amount of 
sunshine, mean temperature, &c., would not be satisfactory, for the 
state of the weather during low water of spring tides is manifestly of 
greater importance to the plant than during any other time. This applies 
to the amount of sunshine, the temperature of the air and of surface 
water (in the case of exposed or nearly exposed plants), and also the 
force of the wind. The effect of the latter on plants in the lower zones 
is much greater at low than at high water. If the wind is moderate 
the movement of the waves is beneficial to the plants in causing effective 
