97 
Cut l evict multifida (Grevi). 
the winter solstice. It is clear that in the case of free 
assimilating plants the light-intensity must be of supreme 
importance for food-supply and increase in bulk, and there 
can be no doubt that it is the abundance of light-supply on 
the ascending part of the temperature-curve which brings 
forward so rapidly the summer vegetation of the British seas ; 
but on the other hand, these same two factors in the Bay 
of Naples appear to lead to the death of the greater number 
of the same plants, and the optimum vegetative period occurs 
there on the descending part of the temperature-curve with 
a diminishing light-supply. 
Berthold 1 , from his observations at Naples, came to the 
conclusion that optimum light-intensity and velocity of the 
current were the main factors, and temperature, although 
important, subsidiary to these. He bases his view chiefly 
on the manner in which the winter and spring annuals last 
on in deeper water or shaded situations ; but it is clear that 
these conditions would also imply a lower degree of tempera- 
ture by affording a protection from the direct heating effect 
of the sun’s rays ; while rapid movements of water, by 
carrying off the heated surface-layers, would produce a similar 
result. In the case of a plant which requires light for assimi- 
lation, it is difficult to isolate the heating effect from the 
light-intensity of the sun’s rays ; and as temperatures are 
easily recorded, it has been thought worth while to collect 
a few of the available data for different localities. As Cutleria 
grows in fairly shallow water, i. e. less than five fathoms, the 
temperatures of the surface-water will be sufficient ; but it is 
to be noted that records of surface-temperatures along the 
shore vary much more than in the open sea, and those alone 
would be absolutely reliable which were taken on the spot 
where the plant was growing : for the influence of the land 
and winds on shallow water in enclosed areas leads to a 
source of error of possibly 3° C. from the average at Plymouth, 
while local variations are still greater in the Mediterranean. 
1 Mittheil. Zool. Stat. Neapel, iii. 1882, p. 293. 
H 
