804 ZONES AND REGIONS [Pt. Ill, Sect. V 



with the least possible employment of heavy material. To these in co- 

 operation, or to the former alone, is due the power possessed by Algae of 

 the plankton of floating permanently. 



The specifically light cell-contents, whose presence is to be regarded as 

 an adaptation to flotation in the Cyanophyceae of the plankton, according to 

 Klebahn 1 , consist of gas-bubbles which look like red dots when seen under the 

 microscope. The fatty oil richly produced by many Algae of the plankton, 

 for instance by Diatomaceae, aids in reducing the specific gravity, without 

 however representing an adaptation for flotation. 



More manifold and more striking are the contrivances for increasing the sur- 

 face. A very simple case is illustrated by Antelminellia gigas(Fig. 492), the 

 giant of its class, which with a volume of several cubic millimeters possesses 

 an extremely fine membrane. The same object is attained in a more perfect 

 manner by outgrowths of various kinds, which often resemble the sailing 

 devices of seeds and fruits, and have reached a wonderful perfection both in the 

 Peridinieae (Figs. 495 and 496) and in the Diatomaceae (Figs. 492 and 493). 

 In various Diatomeae the faculty of floating is attained or increased by the 

 cells cohering. These devices are of course effective only in combination 

 with a specific gravity approximately identical with that of water. They 

 prevent a rapid descent in the water, or the ascent to its surface, that 

 might be occasioned by alterations in weight caused by the products of 

 assimilation. 



The regions of illumination of the plankton resemble those of the 

 benthos. The majority of floating Algae inhabit the topmost layer of the 

 photic region ; but the Oscillariaceae alone give floral tints to the water by 

 means of individuals swimming on the surface. Thus Trichodesmum ery- 

 thraeum, common in the Red Sea, causes the red glimmer to which this sea 

 owes its name. The dysphotic region in all cases is very poor in vegetable 

 plankton, and the aphotic region, at most, may include bacteria. Thus 

 Russell found many bacteria in the Bay of Naples, at a depth of 250 meters, 

 but only very few at 1,100 meters 2 . 



Like the benthos, the plankton exhibits a differentiation into climatic 

 zones. Their boundaries depend on temperature, for cold and warm currents 

 determine them. The Plankton Expedition could distinguish two zones in 

 the Atlantic Ocean, a tropical warm zone and a northern cold one. Their 

 boundary in the western Atlantic is very sharp and coincides with that of 

 the warm Florida current and the cold one from Labrador. In the east, 

 where such pronounced currents do not exist, the transition is more gradual. 

 Each of the two plankton zones is characterized by certain predominant 

 plants. Thus Antelminellia gigas, Gossleriella tropica (Fig. 494), and 

 Planktoniella Sol (Fig. 493) are very characteristic of the warm waters, while 

 Ceratium tripos balticum denotes cold water. 



1 Klebahn, op. cit. 2 Walther, op. cit. 



