l80 PLANT LIFE. 



Thus the plankton must be imagined as present, 

 though invisible, both on the surface of every sea, loch, 

 and river, and on that of the smallest puddle or rivulet, 

 and it must be remembered that it extends to a depth 

 of 8 or 9 feet. ^ It is only polluted streams that are 

 without Algae, and these are generally occupied by 

 swarms of bacteria and fungi. 



(2) The bottom and sides of lochs and the seashore 

 are also generally covered by a dense vegetation, which 

 extends to a depth of about 120 fathoms ; it is unable 

 to go deeper on account of the absorption of light by 

 the upper layers of water. 



On the seashore there are three well-marked zones 

 of Algae. Near the shore, and only covered at high 

 tide, are dense growths of green Algae, chiefly Entero- 

 morpJia and Vaucheria. Somewhat lower down the 

 rocks and stones are closely covered by dark olive- 

 brown kelps and tangles, of which most are species of 

 Fucus. Still lower, and only uncovered at very low 

 tides, are the Red Algae. These are generally small 

 and delicate in appearance, and are often only to be 

 obtained by dredging, or after severe storms, when they 

 have been torn off, and are left stranded at high tide. 

 This distribution depends entirely upon the light. The 

 green species possess the ordinary chlorophyll found in 

 land plants ; the brown have to protect themselves 

 against sunlight at low tide, and to suit themselves 

 to totally different conditions when the water covers 

 them. Hence they have a brown substance in addition. 

 The red are found deep in the water where, apparently, 

 only the dangerous blue rays of sunshine are able to 

 penetrate, and hence they require a specially protective 



^ Halosphaera viridis occurs at a depth of 200 m. in the warmer parts of 

 the Atlantic. 



