100 PLANTS, SEEDS, AND CURRENTS 



plants in the estuaries and coast swamps of tropical regions. An 

 investigation on the lines of that pursued by Prof. Harshberger on 

 the salt-marsh and estuarine plants of the New Jersey coast would 

 yield valuable results (vide Proc. Amer. Philos. Soc, Sept. 1911). 

 For such a study the true mangroves of the genus Rhizophora would 

 offer abundant materials. Though I have made observations on 

 this subject with reference to species of Rhizophora, and particularly 

 Rh. mangle, in different parts of the tropics, they have been discursive 

 in their nature, and, as will appear, they supply rather hints for 

 further inquiry than conclusive and determinate results. 



In Fiji, where I made a special study of Rhizophora mangle, the 

 American species, and of Rh. mucronata, the Asiatic species, I found 

 that whilst they both throve at the coast, only the first named was 

 also at home in the brackish water of the estuaries. This double 

 station, in the coast swamp and in the estuary, was displayed by 

 Rh. mangle in Fiji, in Ecuador, and in Jamaica. In all three localities 

 I ascertained that the tree could live in the higher parts of an 

 estuary where the water was at certain states of the tide quite fresh, 

 and at others brackish or slightly salt. But my data showed that 

 however well it might adapt itself to fresh- water conditions for some 

 hours of each day, it would not be able to live in the parts of the 

 estuary altogether beyond the reach of the sea-water. Typically, 

 Rh. mangle is a tree of the coast swamp and of the mouth of an 

 estuary where the sea- water has in a general sense much of its normal 

 salinity. Its seedlings can be transported by the currents to islets 

 in the open sea, where they give rise to mangrove colonies, such as 

 are presented by the Florida sand-keys. If it is able to adapt itself 

 to the slightly salt or even to the fresh water of the interior of an 

 estuary, it is only for a portion of the day. Rh. mangle is primarily 

 a plant of the typical salt-water swamp of the tropical sea border. 



Yet its adaptability to less saline conditions invites inquiry. As 

 stated in my book on Plant Dispersal (p. 442), it extends in Fiji to 

 the higher reaches of the estuaries, where the density of the water 

 varies according to the state of the tide between 1-000 and 1*010. Its 

 behaviour is the same in Ecuador. In the Guayaquil River it grows 

 forty miles up the estuary, where the water is potable and has a 

 density of 1*000, except at high water, when it is brackish (Ibid., 

 p. 486). In the channels at the back of the city of Guayaquil, to 

 which the sea-water has freer access, the water at high tide had a 

 specific gravity of 1*014; and in response to the increased salinity 

 there was a more typical development of the mangrove formation, 

 Rh. mangle fronting the water with Laguncularia racemosa and 

 Avicennia in the rear. In the case of the Santa Rosa River, which 

 opens on the Ecuador coast near Puerto Bolivar, Rh. mangle, 

 though abundant at the coast, failed altogether about ten miles from 

 the mouth of the estuary, where the water was quite fresh during 

 nine out of the twelve hours, being salt only in the latter part of the 

 rising tide. 



In the Black River of Jamaica I obtained similar results in the 

 month of January. In the company of Laguncularia racemosa this 

 Rhizophora ascends the main channel of the estuary for two and a 



