IRRITABILITY. 529 



The direction of movement can also be affected by chemical 

 means. It is known that zoospores, plasmodia, and Bacterium 

 termo (see Fig. 35, p. 256) move towards free oxygen, and 

 it has been further ascertained that various substances 

 exercise an attractive or a repellent influence. Stahl has 

 observed, in the case of plasmodia, that sodium chloride, 

 potassium nitrate, glycerin, sugar, etc., have the effect of 

 repelling the plasmodia of Aethalium, whereas watery 

 extract of tan attracts them. He has, however, found that 

 the nature of the effect depends upon the concentration of the 

 solution with which the experiment is made. When the 

 water with which a plasmodium is supplied is suddenly 

 replaced by a dilute (0*25 2 per cent.) solution of sugar, the 

 plasmodium at once retires from it. After some days the 

 plasmodium gradually adapts itself to the new conditions 

 and spreads itself out in the solution. If now the solution 

 of sugar be replaced by a more dilute solution or by water, 

 the plasmodium will again retire. 



The most interesting observations of this kind, inasmuch 

 as they have a direct bearing upon an important function 

 of plants, are those of Pfeffer. He finds that the direction 

 of movement of ciliated organisms, such as antherozoids, 

 zoospores of Saprolegnia, motile Schizomycetes, is affected 

 by presenting to them solutions of certain substances which 

 have a specific attraction for these organisms. With regard 

 to antherozoids, Strasburger had pointed out that in Ferns 

 and in Marchantia, they are attracted by the substance 

 which is extruded from the neck of the archegonium. Pfeffer 

 found that malic acid (in combination) is the specific attractive 

 substance for the antherozoids of Ferns and Selaginella, 

 and cane-sugar for those of the Mosses, but he failed to 

 ascertain what the specific substances were for those of the 

 Liverworts, of Marsilia, and of Chara. Extract of meat 

 exercises an attractive influence on the zoospores of Sapro- 

 legnia, and any nutritive solution attracts the motile Schizo- 

 mycetes. 



The experiments were made by introducing capillary tubes filled with 

 the solution into water in which the antherozoids etc. were swimming, 



v. 34 



