HOW TO KNOW THE SEAWEEDS 



immensity of the oceans to a relatively thin layer of illuminated sur- 

 face water and to the narrow intertidal and infratidal fringe within 

 this photic zone. In these places, however, they may be remarkably 

 abundant. 



The general habitats of the plants af this illuminated portion of the 

 sea may be diagrammed as in Figure 1. The vegetable inhabitants of 

 the pelagic division, that is, of the water mass itself, are the phyto- 

 plankton, while those of the benlhic division, or the sea floor, are 

 what we may call the seaweeds, or attached algae. 



The phytoplankton consists of free floating, unattached plants 

 which move about only as their water medium moves. With few 

 exceptions they are unicellular forms of microscopic size requiring 

 quite high magnification to render them visible (Fig. 2). Despite their 

 small size, their habitat in the surface waters of all of the oceans is 

 so vast and their numbers so great that they actually account for 

 more than 95% of the vegetation of the sea. 



Several different kinds of or- 

 ganisms make up the phyto- 

 plankton (Fig. 2) of which may 

 be mentioned the diatoms, the 

 pigmented dinoflagellates, the 

 silicoflagellates, the coccohtho- 

 phores and a few blue-green 

 algae. Most abundant of these 

 constituents are the diatoms, of 

 which several million may 

 sometimes occur in a single 

 quart of sea water. These are 

 unicellular members of the 

 division Chrysophyta whose 

 protoplasm secretes a beauti- 

 fully sculptured, bivalved, si- 

 Hcious shell (Fig. 3). The shell 

 is basically Hke a pill-box in 

 structure, but often is marvel- 

 ously modified for flotation 

 where perpetuation of a species 

 depends upon the ability to re- 

 main in the photic zone. 



Fig. 2. 



Some representatives of the 

 phytoplankton. 



A. A dinoflageiiate, Ceratium. B. A 

 diatom, Planktoniella. C. A coccolitho- 

 phore, Pontosphaera. D. A silicoflagellate, 

 Distephanus. All are drawn to the same 

 scale (X 225) and are shown against a 

 heavy outline which represents the point 

 of a dissecting needle. 



