448 PHYSIOLOGY 
outside the diffusion zone. Besides the reaction of recoil, there are 
accompanying minor reactions which cannot be discussed here. 
Attraction and repulsion. Many different substances have been 
tested with respect to chemotactic control. Some prove to be attractive, 
some indifferent, and some repellent. That responses occur to substances 
that are never met in nature, as well as to those that are not foods, and 
further, that they do not prevent the organisms from coming to serious 
or even fatal injury, indicates that chemotaxy depends upon some 
fundamental property of the protoplasm and is not a mere adaptation 
to secure special ends, however well it may occasionally serve such a 
purpose. In many cases a substance which is attractive at a low con- 
centration proves to be repellent at a higher. In such a case the ques- 
tion arises whether the repellent action is due to the chemical constitu- 
tion of the stimulant or to the osmotic pressure of its solution. As the 
latter seems to be the reason for the action in certain cases, the phe- 
nomenon is named osmotaxy. It has not yet been sufficiently investi- 
gated, but is in many ways parallel to chemotactic irritability. 
Amount effective. The amount of a substance which can act di- 
rectively upon motile organisms is infinitesimal. Thus it was found that 
a minute capillary into which the sperms of a fern crowded, contained, all 
told, less than three hundred-millionths of a milligram (0.000000028 mg.) 
of malic acid. Of this, certainly, only a very small fraction could have 
reached any one of the sperms. Yet relatively the amount is not at all 
insignificant; for the estimated weight of one of the sperms is only ten 
times greater than the total weight of the acid, and if only 1/100,000 of 
the total acted upon a sperm, the ratio would be i : 1,000,000, which is 
still 10 times the ratio of a minimum effective dose of morphin for the 
human body. 
Weber's law. The phenomena of chemotaxy offer an excellent illus- 
tration of a general law of response known as Weber's law. If a fern 
sperm is swimming in water, it will be diverted toward a capillary con- 
taining malic acid whose concentration is i part in 100,000 of water. 
But if it is brought into a solution too weak to evoke a response, say 
i : 200,000, it is so affected by the enveloping acid that it does not respond 
unless the solution in the capillary is 30 times as strong as that by which 
it is surrounded, i.e. 30 : 200,000. If again the concentration of the 
acid in the medium be raised, say from i : 200,000 to i : 100,000, 
the concentration of the stimulant in the tube must be 30 times 
greater, i.e., 30 : 100,000, in order to evoke response; and so on. It 
