284 



PHYSIOLOGY, SYMBIOSIS, ETC. 



Delesseria, Enteromorpha and Fucus is about 25° C. although there 

 is a fairly wide range : at low light-intensities, for example, there is 

 as much assimilation at 5° C. as at 15° C. (cf. fig. 182). In nature 

 the temperature optimum generally corresponds to the temperature 

 attained by the thallus in the sun's rays, whilst Ehrke (1931) also 



100- 



Oi ,' /itg/i U§kt -Sfirin^ 



'' Lou) U^ht- Su.mTTfcr 



I D ' 



/it|A. L^ki-Stinn^ 



Surnrncr > 



' I 1 L 



5 10 15 20 25 30"C 5 10 15 20 25 30 35"C 5 10 15 20 25 30 35°C 



I '2 3 



Fig. 182. Respiration and assimilation in relation to temperature and light. 

 A = assimilation. R = respiration. Experimental period = 3 hours, i = Delesseria. 

 2 = Enteromorpha co?npressa. 3 = Fwcw^ serratus. (After Ehrke.) 



found a correlation between the temperature of maximum assimila- 

 tion and the average temperature of the month of maximum 

 development. 



Table VI 



Optimum temperature 

 for assimilation 



17' C. 

 o"C. 



Average temperature in 



months of maximum 



development 



17° C, Aug.-Sept. 

 0° C, winter and 

 early spring 



Species 



Fucus, Enteromorpha 

 Delesseria 



The principal limiting factor for assimilation appears to be the 

 water content because exposed thalli quickly dry up and cease to 

 assimilate, whilst respiration also sinks very low (cf. fig. 183). For 

 the Fucaceae on a normal cloudy day the amount assimilated during 

 the time they are exposed in 24 hours is only o-7-i-4% of the dry 

 weight, although the fertile tips of Fucus platy carpus acquire a 

 slightly higher percentage. On remoistening, Enteromorpha Linza 

 and Porphyra utnhilicalis take up water at once and very soon 

 commence to assimilate again, whilst the table below shows that 

 the Fucaceae behave in a very different fashion. 



The influence of rain on the Fuci appears to cause a reversible 

 depression of the assimilation rate amounting to 19-25 %, and this 

 factor may assume considerable importance on some coast-lines. 



