116 



immersed in the water. The strands of protoplasm, which were 

 placed in the non-oxygenated water, were seen to grow gradually 

 thinner, until after a time all the protoplasm had crept up above 

 the layer of oil, which, except in excluding the air, had no 

 deleterious effect upon it, to the upper portion of the cylinder, 

 where it could come into contact with the oxygen cf the air. 

 Another method of performing the same experiment is to place a 

 plasmodiurn in a cylinder which is quite full of thoroughly boiled 

 water; to close the opening with a perforated cork, and then to 

 place the cylinder upside down in a plate of fresh water. Very 

 soon the plasmodium is seen to have wandered through the small 

 hole in the cork into the medium which contains oxygen. 



Engelmann (IV. 7) has made some very interesting experiments 

 upon the directing influence exerted by oxygen upon the move- 

 ments of bacteria. He shows that many species of bacteria may be 

 used as a very delicate, test for minute quantities of oxygen. . If into a 

 fluid which contains certain bacteria, a small alga or diatom is 

 introduced it is seen after a short time to be surrounded with a 



dense envelope of bacteria, which 

 have been attracted by the oxy- 

 gen set free by the action of its 

 chlorophyll. 



Verworn (IV. 40) saw a dia- 

 torn quite enclosed by a wall of 

 motionless Spirochsetse whilst 

 the rest of the preparation was 

 quite free from them (Fig. 58). 

 Suddenly the diatom moved a 

 short distance away, getting out 

 of the crowd of Bacteria. The 

 Spirochsetfe, so suddenly left in 

 the lurch by the producer of 

 oxygen, remained quiet for a 

 second, but soon commenced to 

 move about quickly, and to 

 swim after the diatom in dense 

 masses. After a minute or two 

 they had nearly all reassembled 

 round about it, after which they 

 remained motionless as before. 

 This attractive power pos- 



FIG. 59. A large diatom (rinntila.-in) 

 surrounded by a large number of S^ro- 

 chcette plicatilis. (After Verworn IV. 10 

 Fig. U.) 



