270 Bac'dlariece 



filter-papers, but these observations require confirmation, as they do 

 not seem to agree with the researches of Miquel. Castracane 1 has 

 stated that the normal method of reproduction of Diatoms is by 

 spores or germs, and that multiplication by division, although 

 very common, is the exception rather than the rule. This state- 

 ment I consider to be erroneous, as I am fully convinced from my 

 own observations of these plants that the normal method of increase 

 is a multiplication by cell-division. 



Sometimes a resting condition occurs, a pair of new valves 

 being formed within the old ones. This corresponds to the forma- 

 tion of aplanospores in the Chlorophycese, and has been termed 

 the craticular state. 



Diatoms are amongst the commonest of microscopic objects, 

 and they are ubiquitous in all kinds of damp and wet situations. 

 They occur in fresh, brackish, and salt water, often forming a 

 yellowish-brown scum at the surface, or thickly clothing larger 

 Alga3 or other aquatic plants. They are most abundant in cold 

 latitudes, having a decided preference for cold water, although 

 some of them have become adapted to life in hot-springs. In the 

 ocean they are more abundant than any other pelagic plants, and 

 in the cold surface-waters of the Arctic and Antarctic Oceans they 

 occur in prodigious quantity. They form a considerable part of 

 the food of many freshwater and marine animals, and are often 

 found in quantity in the alimentary tracts of Molluscs, Crustacea, 

 Tunicates and Fishes. They are found in abundance in Guano, 

 having passed through the alimentary canals of birds which feed 

 on marine animals. Since they occur in quantity in the surface- 

 waters of the ocean and of lakes, they constitute a very large 

 proportion of both the marine and freshwater plankton. Many of 

 these plankton Diatoms are furnished with long spines or spinous 

 processes, and they are frequently associated to form free-floating 

 colonies. Some genera and species are exclusively pelagic in 

 habit. According to Voigt 2 such forms are sometimes furnished 

 with gelatinous threads and membranes, by means of which their 

 floating capacity is materially assisted. 



Some of the freshwater species are occasionally the cause of 

 foulness of drinking water 3 . This is due to the formation of an oil 



1 Castracane in Annal. de Micrographie, ix, 1897. 



- Voigt in Biol. Ceutralbl. xxi, 1901. 



3 Whipple & Jackson in Journ. New Engl. Waterworks Assoc. xiv, 1889. 



