I9I0.] ON THE DISTRIBUTION OF PLANTS. 461 



centage of substance in solution necessary to cause plasmolysis varies 

 not only with the plasmolyzing substance, but also with the plant 

 used in the experiments. The protoplasm in some plants plasmolyzes 

 quickly ; in other cases with difficulty, so that stronger solutions are 

 necessary to produce a change in the more refractory plants. If 

 we know, therefore, that a certain percentage strength of sodium 

 chloride in solution will produce plasmolysis in say the cells of the 

 staminal hairs of Tradescantia, then if raw or diluted sea water be 

 used and a similar plasmolysis occurs, the percentage of sodium 

 chloride in the sea water must be equivalent to that of the salt solu- 

 tion known to produce similar plasmolytic effects. A comparative 

 table can be constructed by which the varying percentages of sea 

 water can be ascertained. An extensive literature, part of it noticed 

 in the bibliography, is concerned with such plasmolytic studies.'* 



Hydrometric Method. — The use of the hydrometer in determin- 

 ing the salt content of salt marsh soils suggested itself to me, as a 

 simple but efficient method of making a phytogeographic survey of 

 the vegetation of salt marshes upon purely ecologic lines. The 

 advantage of the hydrometer is that it is light, can be carried easily 

 from place to place and lends itself to immediate use, the record 

 depending upon two simple readings, one of specific gravity and one 

 of temperature. The hydrometer is plunged into a vessel contain- 

 ing the water to be tested. 



Styles of Hydrometers. — After a simple hydrometer had been 

 used in a number of preliminary tests, search was made for a hydrom- 

 eter which would record accurately the density of sea water. It was 

 found that there are many kinds of hydrometers in use to test acids, 

 alcohol, alkali, ammonia, bark liquor (tannometer ), beer, benzine, 

 chlorine, cider, coal oil, ether, gasoline, glycerine, milk (lactometer), 

 naptha oil, salt solutions (salimeter). silver solution, sugar, sugar 

 and syrup. Some are constructed with Baume's scales, others with 

 Richter and Trolle's scales and those used to test sugar wath Balling's 

 and Brix's scales. Finally, after testing several different kinds of 



■* Drabble. E., and Lake, H., " The Osmotic Strength of Cell-sap in 

 Plants growing under Different Conditions," The Nezv Phytologist, October, 

 1905, 189; Duggar, B. M., "The Relation of Certain Marine Algae to 

 Various Salt Solutions," Trans. Acad. Sci. St. Louis. XVI., 473-479. 



