PHYSIOLOGY OF THE RED MANGROVE. 643 



given merely illustrate the fact that these aerating or prop roots 

 actually do transpire water vapor and that there is a perceptible 

 difference in the rates of transpiration of trees growing in the com- 

 paratively fresh water of the river and those in the more highly con- 

 centrated salt water of the bay. The average for the series of river 

 tests was 2.37 minutes required to change the indicator in the modi- 

 fied transpirometer, while the bay tests average was 3.66 minutes. 



These prop roots are really aerating roots as Karsten^-*^ and 

 Schimper^-^ and others have shown in their experiments on other 

 trees of the mangrove habit. In the activity of gas exchange as per- 

 formed by aerating roots, there is, of course, considerable moisture 

 transpired. This function of aeration of roots is well discussed by 

 Karsten for the prop roots of Bruguiera eriopetala on experiments 

 which he conducted at the Buitenzorg Botanical Garden. These 

 experiments were very elaborate and were done in the field, for 

 which a cement base had to be constructed in the mud of the swamp 

 and bell jars and glass apparatus fitted on the roots in situ. 

 Manometers were used to regulate pressure and the amounts of 

 COo exchanged in respiration were measured by precipitating it 

 with barium hydroxide as barium carbonate and then back-titrating 

 it with oxalic acid and phenolphthalein. These experiments estab- 

 lished the fact that the roots do function as respiratory organs for 

 definite areas of the plant body and regulate the air supply for these 

 trees whose roots are sunk in the poorly oxygenated and water- 

 saturated mud and slime of the swamp, and they also help to regu- 

 late the fluctuating conditions produced by the tides w^hen part of 

 the tree is submerged, and at other times exposed. Similar experi- 

 ments and observations by GoebeP--"^-^ on Souncratia acida and 

 Avicennia officinalis, and by Schenck^-* on Aviccnnia tomcntosa and 



1-0 Karsten, G., loc. cit, p. 41. 



1-1 Schimper, A. F. W., " Botanische Mittheilungen aus dem Tropen," 

 Heft 3, Die Indo-malayische Strandflora, 1891, p. 2>7- 



1-- Goebel, K., " Ueber die Luftwurzelne von Sonneratia," Ber. dcr Dent. 

 Bot. Gcsell., IV., p. 249. 



1-3 Goebel, K., " Pflanzenbiologische Schilderungen, I. Siidasiatische 

 Strandvegetation," p. 113. 



124 Schenck, H., "Ueber Luftwurzeln von Aviccnnia tomcntosa und La- 

 gimcularia racemosa," Flora, 1889, p. 83. 



