BIOLOGY. 297 



DIATOMS OR BRITTLE WORTS. 



The Diatomaceoe, or Brittleworts, are unicellular microscopic 

 plants, so numerous that tiiere is hardly a spot on the face of the 

 earth, from Spitzbergen to Victoria Land, where they may not be 

 found. They abound in the ocean, in still running fresh water, 

 and even on the surface of the bare ground. 



They extend in latitude beyond the limits of all other plants, 

 and can endure extremes of temperature, being able to exist in 

 thermal springs, and in the pancake ice in the south jDolar lati- 

 tudes. Though much too small to be visible to the naked eye, they 

 occur in such countless myriads as to stain the berg and pancake 

 ice wherever they are washed by the swell of the sea ; and when 

 enclosed in the congealing surface of the water, they impart to 

 the brash and the pancake ice a pale ochreous color. 



Some species of diatoms are so universal that they are found in 

 every region of the globe ; others are local ; but the same species 

 does not inhabit both fresh and salt water, though some are found 

 in brackish pools. The ocean teems with them. Though invisi- 

 ble as individuals to the naked eye, the living masses of the pela- 

 gic diatoms form colored fringes on larger plants, and cover 

 stones and rocks in cushion-like tufts ; they spread over the sur- 

 face as delicate velvet, in filamental strata on the sand, or mixed 

 with the scum of living or decayed vegetable matter, floating on 

 the surface of the sea ; and they exist in immense profusion in the 

 open ocean as tree forms. The numbers in which they exist in all 

 latitudes, at all seasons, and at all depths, — extending from an 

 inch to the lowest limit to which the most attenuated I'ay of light 

 can penetrate, or at which the pressure permits, — are immeasui-a- 

 bly in excess of wh-nt we have been in the habit of assuming. 

 Temperature has little to do with the distribution of diatoms in 

 the tropics ; it decreases with the depth at a tolerably tixed rate, 

 till it becomes stationary. It increases in the polar regions with 

 the depth, and approaches the standard, which is probably uni- 

 versal, near the bed of the ocean. 



Diatoms are social plants crowded together in vast multitudes. 

 Dr. AVallich met with an enormous asscm'olage of a filamental 

 species, from 6 to 20 times as long as it is broad, aggregated in 

 tufted yellow masses, which covered the sea to the depth of some 

 feet, and extended with little interruption throughout 6 degrees 

 of longitude in the Indian Ocean. They were mixed with glisten- 

 ing yellow cylindrical species of such comparatively gigantic size 

 as to be visible to the naked eye. 



Other genera constitute the only vegetation in the high latitudes 

 of the Antarctic Ocean. Dr. Hooker observes that, without the 

 universal diffusion of diatoms in the south polar ocean, there 

 would neither be food for the aquatic animals, nor would the wa- 

 ter be purified from the carbonic acid which animal respiration 

 and the decomposition of matter produce. These small plants af- 

 ford an abundant supply of food to the herbivorous mollusca and 

 other inhabitants of the sea, for they have been found in the stom- 



