436 GENERAL PHYSIOLOGY 



he succeeded macro-chemically in establishing the presence of that 

 acid in all parts of the plant that contain sexual products ; hence 

 the above supposition obtains a degree of probability that borders 

 upon certainty. The spermatozoids of mosses behaved indifferently 

 toward malic acid, but were remarkably chemotactic toward weak 

 solutions of cane-sugar. 



Later, Pfeffer extended his investigations to a large number of 

 Bacteria and flagellate Infusoria, and obtained a series of results 

 that are interesting in the highest degree. These show that very 

 different substances act very differently upon different micro- 

 organisms. Substances to which one species reacts prove ineffect- 

 ive with another. Many substances induce only positive chemo- 

 taxis, others only negative. In the latter case the organism 

 turns away from the source of the stimulus, and the capillary tube 

 remains empty. The threshold of stimulation, i.e., that degree of 

 concentration at which the substances just begin to exert their 

 chemotactic effect, is very different for different substances and 

 different organisms. But the most interesting fact is that many 

 substances that induce positive chemotaxis in weak solution, induce 



O Reixscfavelle Optimum, 



I 1 1 



FIG. 213. Scheme of chemotactic reaction. The concentration increases from the left toward the 

 right ; at the zero-point of concentration, at t the death-point. The arrows indicate the 

 direction of movement. 



negative chemotaxis in the same organisms in strong solution. 

 There exists, therefore, a stimulus-optimum toward which the 

 organisms strive from both sides, from the weaker as well as 

 from the stronger solution. If the solution becomes too strong, 

 death naturally results. Hence, four important grades of concen- 

 tration may be established : the zero-point, where the substance in 

 question is wholly wanting ; the threshold of stimulation, where its 

 concentration is such that the substance is just effective ; the 

 optimum-point, toward which the organisms strive from all degrees 

 of concentration above the threshold ; and the death-point, at which 

 the concentration is too great to permit life (Fig. 213). With the 

 same substance the optimum-point for different organisms usually 

 exists at different grades of concentration. Massart (91) found a 

 beautiful example of this in the different behaviour toward oxygen 

 of a bacterium, Spirillum, and a ciliate infusorian, Anophrys. If 

 the two species of organisms were under the cover-glass in great 

 numbers, both congregated like a wall at the edge of the glass or 

 around air-bubbles, but not immediately at the boundary between 

 the air and the water ; each kept its own distance from the source 

 of oxygen, the Anophrys nearer, the Spirillum somewhat farther. 



