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 i/roo,ooo of 

 the total acted upon a sperm, the ratio would be 1 : 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 1 part in 100,000 of water. 

 But if it is brought into a solution too weak to evoke a response, say 

 1 : 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 1 : 200,000 to 1 : 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 



