510 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



actually in the surface layer carbon dioxide can be replenished from 

 the air, whereas deep down the preponderant tendency is for it to 

 be given off in respiration and decomposition, oxygen being absorbed. 

 So here again the two gases are largely complementary. During the 

 summer and winter periods of relative stagnation, surface waters 

 tend to be more alkaline than deeper layers, owing to the photo- 

 synthetic activity of Algae, which removes the carbonate-ion and 

 leads to a preponderance of hydroxyl-ion. 



As regards temperatures, the surface layer of water already shows 

 much smaller variations than the air lying directly upon it. Con- 

 versely, water has a regulating effect on the temperature of neigh- 

 bouring air-masses, and this effect may operate at considerable 

 distances if the air-masses move over land. The temperatures of 

 the surface waters of the sea rarely if ever exceed 31° C; nor do 

 they fall below the freezing-point of -36° C. Deep down in the 

 ocean the temperatures are commonly rather low and uniform — for 

 example, 06° C. at 2,000 metres' depth in the Antarctic Sea where 

 the surface was 10° C, and i-6° C. at 3,000 metres in the equatorial 

 part of the Pacific Ocean where the surface was 29° C. But in 

 spite of the relatively small amplitude in this respect, so that perennial 

 marine Algae even in cold regions may exhibit no period of winter 

 rest but carry on vegetative activity in summer and reproduce in 

 winter, while in warm seas the difference in temperature is often 

 no longer effective, the floristic organization of marine vegetation 

 depends substantially upon the temperature of the water. Thus the 

 limits of marine floristic regions tend to coincide with particular 

 isotherms, and this is especially true of plankton, whose latitudinal 

 distribution may be related to local temperatures at particular 

 seasons. 



In conjunction with such factors as temperature, the persistence 

 or importation of disseminules is of obvious importance in deter- 

 mining the character of the phytoplanktonic population developing 

 in any one region at various periods of the year. The disseminules 

 may be transported, as adults or otherwise, considerable distances 

 and in quantity by ocean currents, and vast distances will be traversed 

 if a series of generations is involved. Each component type in the 

 plankton has its metabolic requirements adjusted to particular 

 temperature ranges and, having withstood unfavourable seasons 

 elsewhere or in some resistant form, ' blooms ' with the return of 

 suitable conditions, different types succeeding one another largely 

 according to their specific requirements. Even in the yearly 



