1000 



Z800 

 O 



< 



600 



to 

 to 



<400 



— ^ Sunlight 



o — Diffuse light 



18 28 39 47 14 8 14 18 



DAYS DAYS 



Fig. 2i8 Daily drift in assimilation of algae at different tempera- 

 tures in sunlight and diffuse light. A, Fucus serratus winter plant. 



B, Porphyra. (After Lampe.) 



The problem, however, is not as simple as this. Thus Lampe 

 (1935) found that in winter the assimilation rate of Fucus serratus 

 plants rises when it is measured in sunlight under conditions of in- 

 creasing temperature (Fig. 218); on the other hand, in the case of a 

 red alga such as Porphyra, when the temperature is raised above 

 15° C. the assimilation curve is lowered immediately in diffuse 

 light and after seven days in sunlight. This suggests that Fucus is 

 an eurythermal species, tolerating a wide range of temperature, 

 whilst Porphyra is stenothermal, tolerating a narrow range. Com- 

 parable results for Fucus serratus were obtained by Hyde (1938), 

 who found that between 15° and 20° C. the assimilation rate could 

 be increased by raising the Ught intensity, and that there was a 

 certain Ught value (2 x 500 lux) which yielded an optimum in the 

 rate of assimilation. This effect is not observed at low light inten- 

 sities and low temperatures, whilst above 25° C. an increase in the 

 light intensity causes a marked decrease in the assimilation rate 

 (Fig. 219). 



After a period of exposure there is the problem of the return to 

 the normal assimilation rate, because experiments show that the as- 

 similation rate is markedly reduced during exposure. When re- 

 inundated, species such as Ulva linza and Porphyra umhilicalis take 

 up water at once, and very soon are assimilating at their normal 

 rate. The members of the Fucaceae behave in a rather different 

 fashion and their behaviour can be correlated directly with the level 

 they occupy on the shore and the periods of exposure they imdergo. 



402 



