IRRITABILITY 233 



bacteria where only one kind is concerned may be definitely 

 known, the behavior of a swarm of several or many kinds 

 is due partly to the influence of the bacteria themselves 

 upon each other. 



One of the most striking responses to chemical irritation 

 is that exhibited by the tentacles on the leaves of the vari- 

 ous species of the carnivorous Sundew, Drosera. The 

 thorough investigation of this by Darwin* has placed our 

 knowledge on a very definite basis. He found that the ten- 

 tacles would bend over and the leaves curl with a prompt- 

 ness and to a degree proportional to the irritating quality 

 of the substance used. That it was not mere contact that 

 caused the irritation was experimentally demonstrated by 

 placing bits of wood, quartz, etc., and drops of milk, beef 

 infusion, etc., upon the tentacles of expanded leaves. The 

 wood, quartz, etc., did not induce the tentacles to close over 

 them ; the drops of nutrient liquid did. Darwin then tested 

 the influence of a great variety of substances and found 

 that some did, others did not, stimulate the tentacles to 

 close over them. 



The effects of fragrant substances in directing the locomo- 

 tion of animals is a fact of such universal experience that 

 we may well use it as the starting-point of our discussion of 

 chemotaxis. Flies are attracted by the odors of cooking 

 and repelled, as every camper knows, by a smoky fire. 

 Walking, crawling, and swimming animals are attracted or 

 repelled by soluble substances, either in the solid form or in 

 solution. Ants will follow a trail of grains of sugar to its 

 source, but they will be turned from their path by un- 

 pleasant substances placed in their way.f Insects, and 

 doubtless birds also, concerned in the cross-pollination of 

 flowers, are attracted from a distance by the odorous sub- 

 stances formed in them. Only when the animal comes near 

 enough for the flower to be within its range of vision 



* Darwin, C. Insectivorous Plants. On cytological changes in gland 

 cells following stimulation see Huie, L. H., in Quart. Journ. Microsc. Sci.. 

 vols. 39 and 42 ? 1896 and 1899; and Rosenberg. O.. Physiolog.-cyto- 

 log. Untersuch. iiber Drosera rotundifolia. Upsala, 1899. 



f Lubbock, Sir John. Ants. Bees, and Wasps. New York, 1884. 



