THE MOVEMENTS AND SENSITIVITY OF PLANTS 331 



The question presents considerable difficulties, as the same sub- 

 stance, diffusing in the environment of the roots, may produce a 

 double effect on it. On the one hand, it may have a direct influ- 

 ence on the growth rate of that part of the root which comes in 

 contact with the substance. And it may produce also a specific 

 irritation in the highly sensitive root tip. The two effects may 

 be antagonistic to each other. Thus, for instance, a toxic sub- 

 stance affecting the side of the root with which it comes in con- 

 tact, produces a local inhibition of growth and, consequently a posi- 

 tive curvature. The same substance affecting the tip of the root 

 may produce therein a condition of excitation which may be trans- 

 mitted to the growing zone as a stimulus and may result in the 

 formation of a curvature in the opposite direction. It has been 

 possible to separate the two effects by means of a unique method 

 of procedure suggested by Porodko. He placed the seedlings of 

 various plants into a layer of gelatin through which the current 

 of substances producing a chemotropic effect was directed. In 

 some of the experiments, the tip of the root was left outside the 

 gelatin; in others, the growing zone was outside. These experi- 

 ments have led Porodko to conclude that non-electrolytes do not 

 produce any chemotropic effect on roots, while the cations of elec- 

 trolytes, as a rule, have a negative effect, and the anions a posi- 

 tive, the effect of salts being in this respect halfway between that 

 of the cations and the anions. 



Under natural conditions, the role of chemotropism in the direc- 

 tion of growth of roots is of little importance, at least as far as the 

 effect of solutions in the soil is concerned. The distribution of air 

 and water in soil appears to have a much greater effect on the 

 growth of roots. 



If a vessel containing the root system of a plant is hermet- 

 ically closed leaving only one small opening, then the roots will 

 grow in the direction of this opening to meet the current of oxygen. 

 To this phenomenon has been given the name "aerotropism," it 

 being but a special type of chemotropism. Humidity produces a 

 still greater effect on the growth of roots. When it is not equally 

 distributed in the soil, the roots may grow in the direction of the 

 more humid portion. One may observe the bending of roots in the 

 air towards more humid surfaces (Fig. 136). Such an experiment, 

 however, will be successful only in an atmosphere which is almost 

 saturated with water vapor, for otherwise the roots are apt to dry 



