PHYSIOLOGY OF THE RED MANGROVE. 635 



and the concentration percentages as abscissae. That is, the curve 

 indicates the period of time required by the plants to transpire equal 

 quantities of moisture when planted in varying concentrations of 

 water. When growing in fresh water, the plant transpires the unit 

 quantity of moisture in 1.6 minutes, when growing in 100 per cent, 

 salt water, to transpire the same quantity there is required 4.1 

 minutes. The effect then of increasing the salt content is to retard 

 the time of equal transpirations of moisture. The physical law 

 expressing this progressive increase of time interval, necessitated 

 by the increasing concentration, has the mathematical form y = ab^. 

 That is the time, y, for a plant to transpire a unit quantity of mois- 

 ture when the percentage of salt solution is .r, is equal to constant 

 b (approx.^^;) multiplied by a constant, b (approx. = 1.79), 

 raised to x power. For the percentage concentrations used in this 

 work the rate of transpiration then varies directly with the concen- 

 tration. 



The result of these experiments can only in a general way be 

 compared with those of other workers on transpiration, because 

 there are too many factors which were necessarily quite different in 

 the materials and methods. The plants themselves are specially 

 adapted to a water environment and protected against an excessive 

 transpiration, while the ordinarily high salt concentration of the 

 medium of growth makes absorption difficult. The rather high 

 humidity of the air tends to reduce transpiration, while the heat 

 and intense light of their habitat helps to increase it. The general 

 results, however, do correspond with the experiments of Ricome^"' 

 on plants of Malcomia maritima and Alyssu7n maritimum. This 

 investigator cultivated the plants in normal soil and salty soil and 

 transferred to plain Knop's nutrient solution and in Knop's solu- 

 tion to which one per cent, of salt (NaCl) was added. While the 

 general temperatures and humidity were not the same, the light in- 

 tensity was rather diffuse as in the present studies, but the methods 

 of measuring the transpiration differed. Ricome found that both 

 the absorption and transpiration were less in the plants grown in 



10" Ricome, H., " Influence du Chlorure de Sodium sur la Transpiration et 

 rAbsorption de I'Eau chez les Vegetaux," Comptcs Rendiis de I'Acad. dcs 

 Sci. Paris, T. CXXXVII., 1903. 



