Protoplasm is not less sensitive to chemical agents; but its changing and complex nature varies according to the species and its response to the chemical excitant varies also. This response is again a directive orientation. It is called chemotropism. It does not show the absolute precision which one observes with phototropism, for chemical substances are not very regularly diffused.
As an example of chemotropism we may cite the directive orientation produced by oxygen on free lower organisms, animals or plants,-above all, the classic and very pretty experiments made in 1881 and 1894 by Englemann upon Bacterium thermo. This bacterium is very avid for oxygen. When one puts a thin cover-glass on a drop of culture of this species, the bacteria gather at the periphery, where the liquid is in contact with the air. So, also, in a culture charged with carbonic-acid gas, when one introduces a filament of green alga whose chlorophyl, under the influence of light, decomposes carbonic acid and liberates oxygen. If the filament receives the light from a microspectrum the bacteria choose to mass themselves in the radiation absorbed by the chlorophyl; the red radiations in the less refrangible half of the spectrum, the blue and the violet in the opposite part.
Chemotropic reactions play a great part in the lives of insects, and it is believed that they have their seat in the antennae. Thus, the vinegar-fly (Drosophila ampelophila) reacts positively to alcohol, the acids and ethers of fermented fruit. It flies directly to experimental liquids containing these substances. But Barrows has observed (1907) that it takes a manoeuvering method when the terminal segment of one of the antennae has been removed, just as does the mourning-cloak butterfly in the light, when one of its eyes has been covered. In both cases the asymmetry of excitation has resulted in an asymmetry of muscular action. Kellogg describes (1903) a similar phenomenon with the silkworm moth. As soon as it issues, the male seeks a female and succeeds in the dark as well as in the light. What guides it in its search is the emanation from the female perceived by the antennae, and if one of these is cut off the male reaches its object only by describing circles to the side of the remaining antenna. It is led to the mating by a positive chemotropism. According to Loeb, it is a chemotropism of the same kind which directs maggots to the meat on which they feed, and the adult flies toward that on which they lay their eggs.
It is equally well established that insects possess a negative chemotropism. Lubbock declares that ants flee from the essence of cloves and every one can make similar observations concerning other insects. "The backwoodsman (living in the virgin forest) who covers his hands with pennyroyal or who smokes his pipe," says Wheeler, is aware of the chemotropism of the terrible Culicidce (mos-quitos), even if he "is not familiar with the scientific name of the phenomenon."