OSMOTIC CONCENTRATION OF SAP. 209 



That the sap of L. racemosa is profoundly influenced by its 

 substratum is splendidly shown by a determination based on 

 trees growing in practically fresh water on the south shore of 

 Jamaica. An irrigation canal passes through the coastal desert. 

 Trees 1 growing in the marsh beside it gave: 



Jan. 30, A = = 1.64, P = 19.7 



IV. RECAPITULATION. 



The present paper is a contribution to the problem hitherto 

 practically untouched by quantitative methods of the physi- 

 ology of mangrove vegetation. Specifically it deals with the 

 osmotic concentration or osmotic pressure of the leaf tissue 

 fluids. 



Three species, Avicennia nitida of the Avicenniacese, Rhizophora 

 mangle of the Rhizophoraceae, and Laguncularia racemosa of the 

 Terminaliaceae have been examined on the southern shore of the 

 island of Jamaica and in southern Florida. Determinations of 

 freezing point lowering of sap extracted from plants growing in 

 habitats ranging from a substratum saturated with fresh water 

 to the margins of highly saline and practically sterile mud flats 

 have been secured. 



The concentration of the tissue fluids is relatively high through- 

 out. The minimum concentrations are about 20-22 atmospheres 

 for Laguncularia and Rhizophora growing under practically 

 fresh-water influence and 33 atmospheres for Avicennia on the 

 shore of Biscayne Bay. The values may range widely in re- 

 sponse to local conditions. Thus on the southern coast of 

 Jamaica where the plants are growing under the influence of 

 the edaphic and meterological conditions which on higher levels 

 have ' resulted in a splendidly developed desert vegetation, 

 Avicennia shows trustworthy measurements of concentrations 

 up to fifty atmospheres in leaves in a fairly healthy condition. 

 It is quite probable that somewhat higher values will be found. 



1 Determinations on three more or less herbaceous forms were secured for com- 

 parison 



Ammonia latifolia L. April 2, A = 1.60, P = 19.3 



Pluckea purpurascens (Sw.) D.C. April 4, A = 1.21, P = 14.5 



March 30, A = 1.25, P = 15.0 

 Verbesina albaL. (Edipta alba (L.) Hassk.) April 12, A = 1.60, P = 19.2 



