140 DIFFUSION AND OSMOTIC PRESSURE 



A similar reversal of tropism, in this case of geotaxis, was 

 observed in Chromulina woroniniana by Massart. 1 Thus, by 

 osmotically changing the amount of water in the protoplasm 

 the irritability of these organisms can be reversed. 



(2) Osmotaxis. The concentration of the medium acts 

 as a directing stimulus upon the motions of certain free- 

 swimming organisms. This form of response has been 

 named osmotaxis, in analogy to other similar responses to 

 light, heat, chemicals, etc. An organism is said to be posi- 

 tively osmotactic when it swims from the weaker to the 

 stronger solution where these are brought into contact. It is 

 negatively osmotactic when it swims in the opposite direction. 



Since the effect of high concentration of the medium is 

 to extract water from the cell, it will be seen that there must 

 be an identity of nature between this response and that of 

 hydrotropism. An organism is positively hydrotropic when 

 it bends away from a dryer and toward a moister atmos- 

 phere. This phenomenon is exhibited in roots, fungus 

 sporophores, etc. It corresponds to negative osmotaxis, 

 in which the organism swims from a region where water is 

 extracted from its body to one where absorption can take 

 place more freely. Since the conditions under which the 

 two responses are made manifest are so very different, it is 

 probably well to retain the word "osmotaxis." 



Rothert 2 has recently devoted an article to the discussion 

 of this subject. The following facts are mainly derived 

 from this source, Stahl 3 showed that Myxomycete plas- 

 modia, which had become accustomed to a certain concen- 

 tration, would be repelled by any other concentration, either 

 higher or lower. They are thus negatively osmotactic. 



1 J. MASSART, " La sensibility & la concentration chez les 6tres unicellulaires 

 marins," Bull. deVacad. roy. de Belgique, Ser. Ill, Vol. XXII (1891), pp. 148-67. 



2 W. ROTHERT, " Beobachtungen nnd Betrachtungen fiber taktische Reizerschei- 

 nungen," Flora, Vol. LXXXVIII (1901), pp. 371-421. 



3E. STAHL, "Zur Biologie der Myxomyceten," Bot. Zeitg., Vol. XLII (1884), pp. 

 145 ff. 



