3] 



CHEMOTAXIS 



37 



organisms pass only into the first part of the tube ; a^r^, such 

 a balancing of the opposing forces that the organisms stand 

 before the mouth of the capillary tube. 



It will be noticed that the various substances produce dif- 

 ferent effects in the same strength of solution ; and it is 

 interesting to observe (a point to which furtlier reference will 

 be made) that the strength of solution required to produce a 

 given response is roughly proportional to the molecular weight 

 of the substance employed. . 



Inorganic acids and hydrides seem, in general, to act repul- 

 sively, but pliosphoric acid is an important exception to this 

 rule. Dbwitz ('85, pp. 222, 223) states that mammalian 

 spermatozoa are attracted by KHO. 



Organic Compounds. — Alcohol, in grades between 10% and 1%, 

 acts repulsively towards bacteria. Crlycerine is neutral to the 

 same organisms and to zoospores of Saprolegnia. (Stangb, '90.) 

 The sugars, etc., dextrose, milk sugar, dextrin, act attractively 

 vipon Bacterium termo in 10% or weaker solutions. Many or- 

 ganic acids are among the most attractive reagents. It was with 

 malic acid that Pfkffee ('84) tried his earlier fundamental 

 experiments upon the spermatozoids of ferns. The attraction 

 exerted is very great, so that a capillary tube of 0.1 to 0.14 mm. 

 calibre, containing a 0.05% solution of malic acid, attracts 

 from a drop of water full of spermatozoids at the rate of 100 

 individuals in one hour. Even a 0.001% solution acts chemo- 

 tactically. Now, malic acid is of very wide distribution among 

 plants, and it occurs in the fern prothalli upon which the sexual 



