286 INFECTION AND RESISTANCE 



obscure. A large part of our sparse knowledge in this connection,, 

 moreover, has been gained by studies not related to infection. 



The stimuli which determine the motion of cells are, of course, 

 not necessarily chemical, and extensive studies have been made upon 

 the effect of light waves in this connection. Although these inves- 

 tigations are of great biological importance, they have little direct 

 bearing upon the problems of tropism as related to bacteria and leu- 

 kocytes and cannot therefore be considered here. 



Some of the earlier researches upon chemotaxis were those made- 

 by Stahl 24 upon the slime-molds or myxomycetes. These organisms, 

 possess the power of ameboid motion, and were observed by Stahl ta 

 move toward or away from any given region, according to the nature 

 of the substances with which they came in contact, Pfeffer sub- 

 jected this phenomenon to closer analysis. Working with the sperma- 

 tozoa of ferns, swarm spores, bacteria and infusoria, he elaborated 

 an ingenious technique by means of which he was enabled to de- 

 termine directly the negative or positive chemotactic properties of 

 various substances in solution upon these motile forms. His tech- 

 nique was exceedingly simple. Capillary glass tubes, about 8 to 10 

 mm. long and 0.1 mm. in diameter, were sealed at one end in the- 

 flame, and then dropped into a watch-glass. The solution which waa 

 to be tested was poured over the tubes and the watch-glass then, 

 placed under the bell of an air-pump. When the air was evacuated 

 and pressure reduced the tubes became partly filled up with the 

 liquid. They were then removed, washed in water, and placed under 

 a cover slip under which a preparation of the motile cells was swim- 

 ming. Positive chemotaxis was indicated by entrance of the cells, 

 into the tubes, negative, by their refusal to enter. Failure of the 

 solution to exert any chemotactic influence resulted in their moving- 

 into and out of the tubes indiscriminately. 25 



By this technique a large number of interesting observations were 

 made which threw much light upon the causes underlying the move- 

 ments of plant cells. For instance, in investigating the spermatozoa 

 of the ferns it was found that they were attracted strongly by malic 

 acid and its salts, while no other substance investigated approached 

 these compounds in the intensity of positively chemotactic stimula- 

 tion. From this Pfeffer concludes that the bursting of the fern 

 archegonia is accompanied by the liberation of malic acid, this at- 

 tracting the male to the female cell. 



Similar experiments have been carried out since then by numer- 

 ous naturalists, among them Buller, 26 Lidforss, 27 and Jennings, 281 



24 Stahl. Botanische Zeitung, 1884. 



25 Buller. Annals of Botany, Vol. 16, No. 56, 1900. 



26 Buller. Loc. cit. 



27 Lidforss. "Jahrbiicher f. wissensch. Botanik," 41, 1904. 



28 Jennings. "Behavior of Lower Organisms," Columbia Univ. Press.,, 

 Macmillan, 1906. 



