176 PLANT LIFE. 



generally form a series of belts along the margin of the 

 coast or of freshwater, and occupy the ground between 

 open water and ordinary dry ground. 



The Plankton. — The free floating vegetation or 

 Plankton is composed chiefly of minute one-celled 

 Algae, or Seaweeds. They generally belong to either the 

 Diatoms or the Peiddineae, and show very curious modifi- 

 cations for protecting themselves against their minute 

 enemies. The total amount of these minute vegetable 

 forms, and of the animals which feed upon them, varies 

 greatly. The plankton of freshwater lochs, and 

 especially of little bays or backwaters in rivers with a 

 slow current, is exceedingly abundant. It may amount 

 to as much as 60 c.cm., or even to i 10 c.cm. per cm., but 

 varies very greatly according to the particular locality 

 and according to the time of year. The amount of 

 ozone in the water appears to have considerable in- 

 fluence in determining the health of the plankton. In 

 Britain there are two distinct harvests of diatoms, one in 

 spring and the other in autumn. These diatoms are 

 one-cell organisms vvith brown chlorophyll bodies, and 

 they possess a peculiar sort of cell wall. The original 

 cellulose is covered by a deposit of flinty material, 

 which takes the most extraordinary patterns, and is 

 often ornamented by little lines or dots, or raised 

 bosses, showing a bewildering variety of design. This 

 flinty material forms two valves, which may be com- 

 pared to a box and its lid. The lid overlaps the sides 

 of the box. The mathematically exact patterns of 

 their flint valves certainly resemble those of no other 

 living cells. Sometimes the cells resemble a pill-box ; 

 but more often they are, in outline, cigar-shaped or 

 oblong. They have been compared by Kerner to 

 " little protected cruisers." A transparent mucilaginous 



