520 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



very satisfactory and effective method of reproduction that under 

 favourable conditions probably takes place on the average from once 

 to twice every twenty-four hours. It has been calculated that in 

 middle latitudes the increase in total volume of pelagic plants is of the 

 order of 30 per cent, per diem over the year, so that this proportion 

 could die or be removed on the average daily without reducing the 

 plant stock. Whether or not the high latitudes are more prolific 

 than the low ones in the matter of plankton productivity, as used 

 to be contended but is now questioned {see p. 509), there seems no 

 doubt that the total photosynthetic activity taking place in the seas, 

 which occupy slightly over 70 per cent, of the earth's surface, is 

 much greater than that taking place on the land-masses of the 

 world. Actually, the average productivity of similar land and ocean 

 areas appear to be roughly comparable over the year, being said 

 to be of the order of three tons of dry material per acre, though 

 unit volumes of coastal waters are usually many times more productive 

 than those of open ocean waters. 



In arctic regions there are apt to be phenomenal outbursts of 

 Diatoms when the sea-ice melts in early summer. Such ' blooming ' 

 may be associated with the rapid germination of spores previously 

 locked in the ice. The population involved tends to be largely 

 neritic, which is not surprising if we recall that the abundant nutrients 

 and variable salinity and other conditions simulate coastal ones. 

 The cryophytic populations developing on the surfaces of melting 

 sea-ice were described in the last chapter, chiefly on pages 489-91, 

 the component organisms being termed ' cryophytes '. When the 

 ' spring ' production of Diatoms has come to a low ebb owing 

 to marked stratification or incipient exhaustion of nutrients, Peri- 

 dinians (Dinoflagellates) may take over the lead in the matter of 

 organic production in boreal and austral waters — in particular 

 species of Ceratiutn. Their nutrient requirements are lower than 

 those of Diatoms and their rate of growth slower, and they can 

 continue to propagate in impoverished waters, besides which they 

 have the power of locomotion and consequently of adjustment to 

 the best available conditions. Actually, their daily increase under 

 summer conditions is only 30 to 50 per cent., as compared with 

 about ten times as much for some Diatoms. Somewhat similar 

 sequences may be observed in temperate regions. Thus Fig. 171, A, 

 shows an early summer maximum of Diatoms off the Atlantic coast 

 of North America, and Fig. 171, B, shows the later peak of Peridinians 

 with associated animalcules. 



