OCEAN FLORAS 287 



composed of plants suited to grow on rocks or among stones. In fresh waters the 

 temperature of the air exerts the maximum influence on plant-life, whereas in the 

 deep sea that influence is imperceptible within a short distance of the surface. 

 With the exception of a few sea-grasses (Naiadacece), the bulk of the ocean flora 

 is made up of various seaweeds or Algae, some of which possess a growth recalling 

 that of the larger land-plants. The larger seaweeds and all the sea-grasses are 

 confined to the zone within tide-range which extends into the line of surf, the 

 necessary conditions to their existence being bright light and a constant change 

 and movement of water and air. Such as are situated just above the limits of ebb 

 tide, with almost the whole growth under water, are strong and healthy, whereas 

 those placed higher up on the shore are in danger of becoming dried up and are 

 therefore more stunted in growth. In the upper zone the seaweeds are chiefly 

 green, in the middle zone they are brown, while in the lower zone they are mostly 

 red, although brown forms may be found in the upper and green in the middle 

 zones. Some kinds which grow at a considerable depth in the more brightly 

 illumined portions of the ocean thrive also in darker situations near the surface 

 On coral-reefs many seaweeds are restricted to violently agitated water, while 

 others are peculiar to the quieter spots ; but those which grow on sandy or muddy 

 bottoms are essentially still-water types. 



On passing from the temperate to the tropical zones the ocean flora is less rich 

 in variety of form than that of the temperate and frigid zones ; but there are 

 certain marine types of plants exclusively tropical, such as some of the sea-grasses 

 and tree-seaweeds, while the red seaweeds are generally numerous and the brown 

 kinds less abundant. Among the brown seaweeds is, however, the well-known 

 yellow Sargassum, found in the tropics at some distance from land floating on the 

 surface of the sea, particularly in the tropical Atlantic, where it forms the so-called 

 Sargasso Sea. Other species of the same group occur in the tropics, although 

 most of these also range into the adjacent temperate regions. The scanty flora of 

 the bed of the tropical oceans is still very imperfectly known, especially when 

 compared with those of temperate waters, some of which present peculiarities by 

 no means easy of explanation. The flora of the Australasian seas, for example, is 

 quite as singular and unique as are the fauna and flora of the mainland ; while the 

 seaweeds of the Red Sea differ in toto from those of the Mediterranean. As a 

 good example of the flora of the warmer temperate waters, that of the Bay of 

 Naples may be selected. Here at low-water mark the sea-bottom is muddy and 

 sandy with a substratum of rocks and stones ; and upon this flourish luxuriant 

 forests of red and green seaweed, which in deeper waters become either wanting or 

 but poorly developed. The deeper zones possess, however, a rich plant-growth of 

 their own, the sea-bed off Capri at a depth of from 60 to 65 fathoms having a 

 luxuriant vegetation, while a stretch lying at a depth of about 30 fathoms is 

 covered with a regular meadow of sea-grass, whereas in from 40 to 58 fathoms of 

 water vegetation is altogether lacking. Near the surface plant-growth is strongest 

 in winter and spring, while in the depths it is fullest in summer and autumn. 

 Localities, which in winter are covered with a luxuriant growth, in summer are 

 bare, and during the latter season many plants which belong to the upper zone 

 retreat to the depths, a few even differing somewhat in form at the two seasons. 



