THE ACTION OF SOME DENITRIFYING BACTERIA. 140 



directly put to the test. On the other hand, the existence of denitrify- 

 ing bacteria in temperate waters has long been known, and it would 

 seem a fair deduction that should this Ijacterial destruction of nitrates 

 take place with greater intensity and completeness in tropical than 

 temperate waters, an explanation of the relative scarcity of plankton 

 in the former would be otfered, and it was with the object of investigat- 

 ing this question that the present work was undertaken. 



My thanks are due to the Marine Biological Department of the 

 Carnegie Institute of Washington, U.S.A., for their kindness in 

 accommodating me in their Laboratory at Loggerhead Key, Dry 

 Tortugas, and at their temporary Laboratory in Port Eoyal, Jamaica, 

 B.W.I., and also to the Marine Biological Association of the "United 

 Kingdom for giving me facilities for work in their Plymouth 

 Laboratory, 



METHODS. 



At the Dry Tortugas Laboratory, the motor-yacht Anton Dohrn 

 enabled me to obtain samples of water from the middle of the 

 Gulf Stream, and a number of smaller motor-boats were always 

 available for shorter journeys. At Port Eoyal, Jamaica, it was 

 necessary to depend on a sailing-boat, but owing to the remarkable 

 regularity with which a breeze springs up every morning, no difficulty 

 was encountered from this cause. At Plymouth the s.s. Oithona 

 enabled me to obtain samples of water from a point seventy miles west 

 of Ushant. 



The observations were made at Port Pioyal during May, at the 

 Tortugas during June, and at Plymouth in August, 1911. 



For purposes of comparison an endeavour was made in every case to 

 obtain samples of water from localities where truly oceanic conditions 

 prevailed, and hence samples were not obtained from the English 

 Channel, where previous experience had shown that tlie bacterial flora 

 was abundant and varied, owing to contamination from the land. 



Surface samples were collected in sterilized wide-mouthed stoppered 

 bottles holding about twelve ounces. Care was taken to avoid con- 

 tamination from the sides of the boat by always collecting the 

 samples from the bows when the boat was going ahead. 



Deep samples were collected at the Tortugas in retort-shaped 

 glass flasks of about 300 c.c. capacity, with narrow, recurved, long- 

 drawn-out necks. These were sterilized, exhausted, and sealed : they 

 were then lowered in an apparatus in which the extremity of the neck 

 could be broken off at any desired depth by sending a messenger down 

 the sounding wire, when the flasks became completely filled with 



