290 POPULATIONS OF THE SEA 



provements of the media, the knowledge of the need for these or- 

 ganic compounds and the success with the isolation of bacteria- 

 free cultures seem to promise that a new era has been initiated in 

 experimental studies of the autecology of the marine phyto- 

 plankton. 



There are also other indications that a more satisfactory basis 

 for such work can now be established. I refer to recent work on 

 the life cycle of the plankton algae. The species, which would be 

 most suitable for culture work and studies on the influence of 

 environmental conditions on their occurrence, are those which 

 reproduce by binary fission, without any complications in the 

 form of sexual reproduction or cyst formation. Among the more 

 important groups of plankton algae, the dinoflagellates seem to be 

 closest to this ideal. None of the marine brown dinoflagellates 

 which have been cultured, representing species of Amphidinium, 

 Ceratium, Exuviaella, Goniaulax, Gymnodinium, Gyrodinium, Peri- 

 dtnium, Prorocentnim, and Protoceratium, has shown any sign 

 of sexual reproduction. Some of them form cycts when the cul- 

 tures become too dense or old, but there are no difficulties in keep- 

 ing the cultures in active growth without cyst formation. 



The diatoms, on the other hand, are not so easily handled in 

 culture work. Their special mode of cell division, whereby one of 

 the daughter cells becomes smaller than the other, leads to a varia- 

 tion in cell size w^ithln a population. Transitions from narrow to 

 broad cells take place through auxospores, in connection with 

 sexual reproduction. In species where auxospore formation takes 

 place readily, the average cell size may not vary excessively, while 

 in other species, which form auxospores at longer intervals, popu- 

 lations wdth very different values for the average cell size may oc- 

 cur both in nature and in culture. As the growth rate is likely to 

 vary with cell size, more extensive experiments must be carried 

 out in order to obtain results w^hich are applicable for interpreta- 

 tion of field observations. For the culture work an intimate knowl- 

 edge of the life cycle of the species in question and the conditions 

 of sexual reproduction are therefore of great value. Through the 

 work of von Stosch (1954) the methods for life cycle studies on 

 centric diatoms, which are predominant in the sea, have been 



