i6o HYDROPHYTES sect, iv 



of Iceland and Greenland ; in April and May there is another rich diato- 

 maceous plankton, whose species demand somewhat higher tempera- 

 tures ; in June and July, a less rich and more uniform diatomaceous 

 plankton (with Rhizosolenia alata) ; from August to November, the 

 warmest season, there is a rich plankton of Peridineae, often with an 

 admixture of a diatomaceous one, which is rich in species and hkewise 

 occurs on the southern coasts of the North Sea ; finally, in December 

 and January there is a poorer plankton, consisting of remnants of the 

 preceding. Most investigators assume that in the case of coast-forms 

 the spores, which are mainly produced at the conclusion of the vegetative 

 periods of the respective species, sink to the ground, and rest there until 

 the commencement of the favourable season. 



In fresh water lakes the seasonal changes are not less remarkable 

 than in seas : this is shown, for instance, by the illustrations in Wesen- 

 berg-Lund's great work (1905).^ 



Associations of the open sea are more stable, as the voluminous 

 masses of water in the ocean do not experience such considerable changes. 

 In general the amount of plankton is small ; yet, on the one hand, plank- 

 ton sometimes occurs in such abundance as to cloud the water, while on 

 the other hand, it is meagre where different kinds of currents impinge. 



False plankton (Tycholimnetic plankton) is formed in fresh water 

 by various Chlorophyceae, including Zygnema, Mougeotia, Spirogyra, 

 and others, which at first are fixed, but subsequently break loose, and, 

 owing to the entanglement among their threads of gas that they have 

 evolved, they rise to the surface of the water and float there. Other 

 plants, such as Sargassum in the sea, that are normally fixed, but 

 become detached by constant movement of the water, may also be 

 placed in this category. 



Geographical distribution. Of the geographical distribution of plankton- 

 organisms little is yet known. In this respect we are best acquainted 

 with the temperate Atlantic Ocean, North and Baltic seas, and the sea 

 surrounding Iceland. Diatomaceae play the greatest part in the colder 

 seas, Cyanophyceae in the tropics, and Peridineae in temperate and 

 tropical regions. The species throughout have a wide area of distribution, 

 because the external conditions may be the same over such vast expanses, 

 and in widely separated spots ; yet each region has a characteristic 

 flora.2 



SUB-FORMATIONS 



Plankton may be classified into the following three sub-formations : 

 Haloplankton, salt-water plankton. 

 Limnoplankton, fresh-water plankton. 

 Saproplankton, foul-water plankton. 



A. Haloplankton. Salt-water Plankton. 



The plankton of salt water may be subdivided into neritic and oceanic 

 haloplankton. 



Neritic plankton is confined to the coast ; in the tropics it consists 

 of Diatomaceae, Cyanophyceae, and Peridineae, but very little is known 



^ See also Kofoid, 1903, 1908. 



^ The recent literature is cited in papers by G. Karsten, 1898, 1905-6, 1907. 



I 



