5l8 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



of long spines or other projections to resist sinking. One or more 

 of these features leading to increased form-resistance is commonly 

 found in planktonic Diatoms, which, it is interesting to note, also 

 tend to have lighter cell-walls than their benthic counterparts. 

 Peridinians, in spite of their power of locomotion, may also have 

 a parachute-like or winged form, or may develop marked asymmetry 

 that leads to orientation of the sinking body so that the long axis 

 lies horizontally, accordingly providing the maximum surface and 

 resistance to passive sinking. It is also interesting to note that, in 

 keeping with the reduced viscosity of warmer waters, the summer 

 forms of Diatoms tend to have lighter shells and the southern 

 Peridinians larger projections than their colder-water counterparts. 



The majority of marine phytoplanktonic organisms inhabit the 

 euphotic zone, and especially its upper layers. The dysphotic 

 zone is commonly very poor in phytoplankton, and the aphotic 

 zone, except for occasional stray photosynthetic individuals, is 

 limited to saprophytic, parasitic, or chemosynthetic forms. Seasonal 

 variations may also be very marked in marine phytoplankton, often 

 including both spring and autumn maxima when nutrients are 

 plentiful and light and temperatures allow rapid development. Quite 

 apart from this, the seasons may produce a sequence of types favoured 

 by, or able to withstand, particular conditions. 



Owing to the minute size of many of the organisms, only a small 

 proportion of the total phytoplanktonic population present is satis- 

 factorily secured by the collecting nets usually employed. Con- 

 sequently concentration by settling, centrifuging, or filtration is 

 necessary for proper appraisal. The exceedingly minute material 

 thus obtained is called ' nannoplankton ' and includes the smaller 

 Diatoms and Peridinians, Bacteria, numerous flagellates such as 

 Coccolithophores which may be no more than 5 microns in dimen- 

 sions, and of course the tiniest animalcules. 



The composition and density of planktonic vegetation at any 

 particular place and time naturally depend upon the intensity and 

 interaction of various factors. Among the more obvious of these 

 are the rate of reproduction and the rate of removal of individuals 

 by death or sometimes fusion, by consumption by other organisms, 

 or by removal through sinking or in water-currents. There is also 

 a dependence on the rate of growth which itself depends upon the 

 size and type of the parent stock, upon the intensity of the light 

 falling on the surface and reaching the particular depth, and upon 

 the concentration and availability of the elements which are essential 



