180 CARNEGIE INSTITUTION OF WASHINGTON. 



These observations indicate, therefore, that the larger amount of nitrogen 

 is actually responsible for a richer plankton in the cooler waters as compared 

 with the warmer. It seems established, also, that an important limiting 

 factor in the distribution of Mayorella conipes, and perhaps other species of 

 amebas, is the food-supply. Similar experiments on a larger scale and for a 

 longer time would probably show that the food-supply is the most important 

 limiting factor under natural conditions controlling the distribution of 

 amebas. 



This method of changing the plankton constituents is very simple and must 

 be of value in the investigation of the distribution and life cycles of many 

 animals depending upon plankton organisms for food, especially mollusks 

 and fish, not only in salt but also in fresh water. The low cost of nitiate makes 

 it possible to perform such experiments on a large scale on natural bodies of 

 water. 



A study was made of floating Sargassum to determine to what extent the 

 larger species of amebas (i. e., those that feed on organisms other than 

 bacteria) are found on it. Floating Sargassum was collected at a number of 

 stations several miles from Loggerhead Key, and then shaken with a small 

 quantity of water to wash off the amebas and other animals clinging to it. 

 In the sediment from the washings only one species of ameba was regularly 

 present, Trichamceba sphcerarum, and it was found only in small number. 

 These amebas are brilliantly colored on account of the endoplasmic inclusions. 

 A large number of roundish bodies, some of brilliant orange, others of a bright- 

 green color, were always present. The nature or origin of these bodies was 

 not determined. I noted, however, that a large proportion of animals asso- 

 ciated with Sargassum, including ciliates and flatworms, also contain large 

 numbers of similar highly colored bodies. The Trichamceba sphcerarum of 

 Sargassum is larger than that found growing in the surface ooze on muddy 

 bottoms and differs somewhat in appearance from the latter. That both 

 kinds belong to the same species is indicated, however, by the fact that they 

 react in the same way to the various dilutions of sea-water ; that is, they assume 

 a spherical shape in 75 per cent sea-water and die in 50 per cent sea-water. 



A Reconnaisance of the Vegetation of Tahiti, with Special Reference to that of 



the Reefs, by William Albert Setchell. 

 In pursuance of the plan to make a reconnaisance of the vegetation of the 

 island of Tahiti, with special reference to the marine vegetation of the encirc- 

 ling reefs, headquarters were established at Papeete and maintained there 

 from May 16 to July 19, 1922. My wife, Clara Ball Setchell, and Harold 

 Ernest Parks, technical assistant in botany in the University of California, 

 gave the utmost assistance in carrying out the details of the project. The 

 lower levels of the main island were explored in large part, as well as a portion 

 of both the east and west shores of the peninsula of Taiarapu. A hasty trip 

 was undertaken to the adjacent island of Moorea for purposes of reef com- 

 parison. Certain of the reefs about Tahiti were selected as types and were 

 investigated as intensively as time and facilities allowed. The land flora was 

 collected and ecological conditions noted up to about 3,600 feet. Rock, soil, 

 and coral samples were selected as bearing on the general and special problems 

 and climatic data were assembled so far as recorded. The resulting notes and 



