PHYSIOLOGY 



287 



rate considerably (of. fig. 182). The assimilation of any alga ex- 

 hibits a well-marked optimum which depends upon both light and 

 temperature : at low temperatures the optimum occurs at low light 

 intensities, and this is obviously significant in the case of those 

 algae that grow in cold waters. Those algae which behave in the 

 same way as terrestrial shade plants build up food reserves during 

 the cold time of the year when conditions are favourable for them, 

 and then lose the material during the more unfavourable warmer 

 periods, whilst in summer time those algae which behave like 

 terrestrial sun plants have a high assimilation rate (cf. p. 359). 



/^ssim. 



1000 



^ Sunlldh.t' , , 



B 



12 5 9 18 26 28 39 47 12 4 8 14 18 26 29 35 

 Days Days 



Fig. 185. Daily drift in assimilation of algae at different temperatures in sunlight 

 and diffuse light. A, Fiicus serratus, winter plant. B, Porphyra. (After Lampe.) 



More recently Lampe (1935) has re-emphasized the fact that the 

 relation between temperature and distribution is not completely 

 solved, and furthermore he stresses the fact that it does not appear 

 to be wholly dependent upon a physiological basis. In winter, the 

 assimilation rate of Fucus serratus plants is found to rise when it is 

 measured in sunlight under conditions of increasing temperatures 

 (cf. also Ehrke above) ; 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 difltused light and 

 after seven days in sunlight (cf. figs. 185, 186). From this it may be 

 concluded that Fucus is an eurythermal species, i.e. tolerates a wide 

 range of temperature, whilst Porphyra is a stenothermal species, 

 i.e. tolerates only a narrow range of temperature (cf. also p. 367). 



