34 CHEMICAL AGENTS AND PROTOPLASM [Cn. 1 



Returning now to the simple organisms, let us consider the 

 kinds of chemical substances which incite to a response. 



Oxygen is for almost all organisms a means of attraction. 

 Various methods of demonstrating this have been used. Thus 

 STANGE ('90, p. 139) filled capillary tubes with pure oxygen, 

 under an air-pump, and brought them to the water containing 

 zoospores, which then penetrated into them. 



The aggregation of zoospores and bacteria to the edges of 

 the cover-glass, to the open end of a capillary tube (ADEKHOLD, 



Air bubble 



/# 



, Zone of Spirillum **' ;V 



Zone 0/ Anophrys . . *' * 



V^ 35 ^ A 



Gt,. V V.-:V:^ C . T .::;V^ 



FIG. 2. a. Corner of the glass slip covering a drop of liquid containing Spirillum 

 and Anophrys, showing their aggregation with reference to the aerated bound- 

 ing film of the drop. b. An air-bubble in the drop, showing aggregation of the 

 organisms about it. (From MASSART, '91.) 



'88, p. 314), or to an enclosed air-bubble, are well-known phe- 

 nomena. (Cf . MASSART, '91, p. 159 ; VEEWOEN, '89, p. 107 ; 

 and see Fig. 2.) 



ENGELMANN ('94) has employed a still more refined method 

 of studying attraction towards oxygen. A drop of foul water 

 is put on a glass slide with an alga cell in the centre, and is 

 covered by a cover-glass whose edges are hermetically sealed 

 by vaseline. The bacteria are uniformly distributed in the 

 water, moving in a lively manner, since they gain oxygen 

 everywhere. If the slide thus prepared is kept in the dark, 

 the oxygen is gradually consumed and the bacteria become 

 quiescent, showing no distribution with reference to the cen- 

 tral chlorophyllaceous body (Fig. 3). 



If the slide is now exposed to the light, oxygen is produced 

 by the alga and a regular distribution of the bacteria in two 

 distinct regions in a mass around the central alga, and in a i 



