CHEMOT'ROPISM ( from Gk. xripeto, che a leheilly truer, a turning, from rphretv, trepein, to turn). The sensitiveness of certain plant-organs by virtue of which they change the direction of their growth when acted upon by chemical substances. if an organ bends so as to grow toward the source of a sub stance, it is said to lie positively ehemotropie to substance; if it turns away from it, it is negatively chemotropie. The organ tends to place itself so that it shall be equally stimulated on all sides by the diffusing chemical substance. In elongating organs, such as roots. fungous fila ments, etc.. the reaction is one of growth; the curvature is brought about either by the retarda tion of growth on one side or by its acceleration on the other, or by both together. Of course the sole on which retardation occurs becomes con CSTe; the opposite ids convex. If a root is posi tively acinotropic toward a substance, that side which comes in contact with the most of the sub stance —as the latter diffuses through the medium in Which the root is growing—will lw retarded in growth and become concave. if the root is negatively chemotropie, aeceleration of i...rowth will take the place of this retardation, and that side will become convex. Thus the root tip is turned either toward or away from the source of the diffusing compound, and the bend ing continues until all side: are equally stimu lated. What the mechanism of this response is
has not been made out. It is probable that the difference in absorption of the stimulating sub stance on the two sides is accountable for the change in growth. but further than this we can not go. The reaction is seen in case of many gases, liquids, and solids when dissolved in water. The term (q.v.) has been applied to the power of responding to gases in this way. A good example of chemotropism is furnished by the following: If fungi be grown in a plate of moist gelatin, and an excess, of sugar be added to a certain part of the gelatin plate. the filaments will bend and from all parts of the medium toward the part which is richer in sugar. The diffusion of the sugar from this re gion out into the surrounding material is the oc casion of the response. Also, certain fungi which grow upon decaying meat (e.g. Saprolegnia) are similarly attracted by meat extract. Another example occurs in the fly - catching sundew (Droscra). The tentacles upon its leaves ex hibit chemotropism toward nitrogenous com pounds and toward ammonium phosphate to a very marked degree.