(562 BOWMAN— ECOLOGY AND 



instrument is filled with alcohol. When the bottom or any desired 

 depth is reached a heavy weight is sent down the slender wire cable 

 which attaches the instrument to the boat. This weight operates a 

 clip which releases the spring and the alcohol is allowed to escape 

 and the sea water flows in, another weight is sent down which falls 

 on a different clip and the instrument is closed, and the sample 

 drawn up in a few seconds. In these experiments, alcohol was not 

 needed ; as the depths in the river at no place exceeded nine feet, 

 and as the launch was used in place of the yacht, the instrument 

 was lowered by hand instead of the sounding machine. Samples 

 were taken then from both the top and the bottom layers and hy- 

 drometer readings made of them. The readings were begun at 

 9.15 A.M. on the outer side of the harbor, in the Atlantic, one 

 quarter mile off shore, the second made just off shore near the 

 mangroves growing on the outer side of the Harbor near the Gov- 

 ernment Channel. Then another halfway across the Bay (Bay 

 Biscayne) and the next at the mouth of the Miami River, from 

 thence every half mile up stream until Rhisophora no longer ap- 

 peared. These readings are tabulated below. 



By these readings it is seen that there is a very decided difference 

 in the sahnity of the top and bottom layers. The distribution of 

 the mangroves is also correlated with the comparison of the top 

 and bottom readings and we may infer that the salt of the water 

 which is carried up by the under-current is the factor in the physio- 

 logical relations of the plant which compose the optimum conditions 

 for its growth and as this decreases in concentration by dilution with 

 the downstream current, the Rhizophora fringe gradually dwindles 

 and disappears. 



Dimorphism. 



Before leaving this chapter on ecology, a word may be said 

 regarding dimorphism in the genus, Rhizophora. Guppy has re- 

 corded very obvious dimorphism in R. mucronata in Fiji and in R. 

 mangle in Ecuador, the double form in the first consisting of a 

 fertile type and a sterile (selala) type in which the pollen is not 

 viable. The second species consists in Ecuador of the " mangle 

 grande " and the " mangle chico," a tall and a little mangrove. The 



