76 PLANT-LIFE 



inflations in the branches, and occurs between half- 

 tide level and high-water mark. Associated with 

 Knobbed-Wrack, but extending beyond it to below half- 

 tide level, is Notched- Wrack (Fucus serratus, Fig. 25). 

 This species has a thallus, with a midrib, and which 

 forks repeatedly in its branching. The margins are in- 

 variably toothed like a saw — i.e., serrate. The colour 

 is dark olive-green, and the length varies from 2 to 6 

 feet. At low-water mark, and considerably beyond, 

 we discover the big Tangles, with their long, flat fronds. 

 The region in which this sea-forest thrives is called the 

 Laminarian Zone. 



Seeing that animals are fundamentally dependent 

 upon plants for their nutrition, it is plain that Seaweeds 

 are all-important as a food basis for marine animals. 

 Even in the vast ocean regions where ordinary Seaweeds 

 do not occur, there is a floating plankton of very minute 

 plants, which are, perhaps, even more than the grains 

 of sand on the seashore in number. Diatoms are the 

 most important members of this floating flora in the 

 colder seas of the north and south, and they are found 

 mixed with tiny Peridinece in the temperate seas. They 

 have a much wider range than this note indicates. The 

 Peridinece are one-cell Algse, with curious cellulose walls, 

 made up of close-fitting, angular plates. The wall is 

 traversed by a horizontal groove, in which there is a 

 pore. Through this opening two slender flagella issue; 

 one lies in the groove, and the other is extended freely 

 into the water, and propels the organism by lash-like 

 movements. Some species emit light, and are the chief 

 agents in the luminosity of the water in certain areas of 

 the sea. 



