THE DISTRIBUTION OF ALGAE
From what has been said already it will be realized that, while the Chlorophyceae occur chiefly in fresh \yater, the Phaeophyceae and Rhodophyceae are predominantly marine Algae. It has been shown that there appears to be a direct correlation between the conditions of life and the colour of the pigment. Green Algae can only assimilate 'when growing in bright light, while the Brown Algae can do with rather less, and the Red Algae with least of all.
From the types we have described it is clear that members of the Chlorophyceae occupy a great variety of situations. Quite apart from fresh water, which is their most typical habitat, certain groups live exclusively in the sea, while others, such as Cladophora, adapt themselves readily to either salt or brackish water, to life on salt marshes and similar places. Recent investigations have shown that many Algae can be isolated from the soil, and it seems clear that many of the unicellular types live primarily in damp soils.
One of the most interesting aspects of algal distribution, however, is the zonation to be seen on the seashore. If a study be made of the succession of types met with, from high tide mark to well below the lowest levels of the spring tides, it will be found that the genera and species will generally appear in the same order, provided other factors are the same. Exposed on the surface of rocks, only covered by the spring tides, will be found the Channelled Wrack (Pelvetia canaliculata) and the Spiral Wrack (Fucus spiralis).
Somewhat lower down on these rocks the Knobbed Wrack (Ascophyllum nodosum), the Bladderwrack (Fucus vesiculosus) and the Serrated 'Wrack (Fucus serratus) form a zone. These Algae are only immersed at high tide and have to withstand considerable exposure to sun and rain.
As we pass further down between tide marks we find that various Red Algae make their appearance, Laurencia pinnatifida, Gigartina stellata and Corallin a officinalis being the most common and making another recognizable zone. At about low tide mark species of the Kelps or Tangles (Laminaria digitata and Laminaria saccharina) appear; sheltering under their great fronds are many small Brown and Red species. Slightly lower still the Seathongs (Himanthalia lorea) appears, with its great thong-like branches sometimes 6 ft. long, and the Badderlocks (Alaria esculenta), with a broad frond sometimes as much as 6 ft. in length. Lowest of all the Brown,.algae comes the giant Sacconhiza bulbosa, growing in several fathoms of water and producing thalli more than 12 ft. long.
Such plants when growing in large numbers cast a gloom through the water, and among the fronds there is only sufficient light for members of the Rhodophyceae to live. In the forests formed by these Laminarias every rock and cranny is covered by small Red Algae. Only these Algae can live and thrive under such conditions, by virtue of the red pigments which they possess, which are supposed to enable them to absorb the predominating bluish light under water. \file need not enumerate here the various species which occur, but we must realize that under these conditions they are much more luxuriant than the stunted specimens we find sheltering under Fucus plants on the higher zones of the shore.
It has been observed that certain species of seaweeds show a tendency to move downwards tovvards lower zones of the shore during the winter months, while others, in contrast, migrate landwards.
Quite apart from the larger Algae, whose distribution we may study on the seashore or the lakeside, there are countless microscopic species, which contribute to that floating population of organisms collectively spoken of as plankton. Many of these species multiply at a surprising rate, sometimes covering the whole surface of a pond in a few days with an almost pure culture of some particular organism, only to disappear again as quickly as they have come.
Many and complex are the problems associated with the distribution of the Algae, but sufficient has been said to indicate the scope of the subject and, it is hoped, to stimulate the reader to further observations of his own.
