626 Sir John Mur-ray [Feb. 7, 



the phanerogams are represented only by the eel-grass {Zostera 

 marina), which is very common in certain localities, while the vast 

 majority of sea-plants belong to the algffi. Unlike terrestrial plants, 

 the marine alg« find their nonrishment dissolved in the sea-water 

 and uniformly distributed around them, so that they can take in food 

 throughout the entire surface of the organism : they all possess green 

 chlorophyll — that magician which, conjuring with the sunbeams, is 

 able to build up organic compounds from inorganic constituents. 



The marine algae have been arranged into four groups according 

 to their predominant colouring : (1) ChIorophycea3, or green algfe ; 

 (2) Cyanophyceaj, or blue-green algse ; (3) Pliasophyceae, or brown 

 algse ; and (4) Ehodophyce^e, or red algse. 



The attached brown algse are the best known of marine plants, 

 and include the Fifcus and Laminaria of the shore and shallow water, 

 the gigantic Macroei/stis, sometimes with stems 700 or 800 feet in 

 length, and the famous Gulf weed {Sargassimi), which, though found 

 in great floating banks in the Sargasso Sea, is derived from the coasts 

 of the West Indian region. The attached red algte include delicately 

 branching forms, like the PolysipJionia of the English coasts, and 

 massive forms encrusted with calcareous matter (nuUiporcs or coral- 

 lines), which play an important part in the building-up of some coral 

 reefs and in protecting rocks from erosion. 



The floating algse (or Phytoplankton) are all of small size, mostly 

 indeed of microscopic dimensions, and till recent years were very 

 little studied. They occur throughout the photic zone in great pro- 

 fusion, especially in the sub-surface layers, building up the organic 

 substances upon whicli all marine animals depend for food. They 

 include a few representatives of green and blue-green algai, but con- 

 sist mostly of diatoms, peridineans, brown and calcareous flagellates. 



Diatoms are found in fresh water and in damp places as well as 

 in the sea, where they occur both free in the water and attached to 

 other algge and animals : the cell-walls are siliceous and often 

 beautifully sculptured. They have been arranged in four groups, 

 according to their type of structure : (1) the bladder type, like 

 Coscmodiscus ; (2) the ribbon type, like Fragillaria ; (3) the hair 

 type, like Rhizosolenia ; and (4) the branching type, like Ghoitomras. 

 They vary in form, and develop special suspension-organs, in re- 

 sponse to variations in the viscosity of the water, and in coastal 

 regions they form resting-spores which sink to the bottom or into 

 deep water and lie dormant during unfavourable seasons of the year. 

 The oceanic diatoms occur most abundantly where the water is 

 relatively fresh and loaded with suspended matter, this distribution 

 being correlated apparently with the amount of colloid silica or clay in 

 the water. The dead frustules accumulate on the ocean-floor in the 

 Southern Ocean and in other regions in such numbers that the 

 deposit is called diatom ooze. 



Peridineans are found in abundance and in a variety of forms, 



