GROWTH OF PLANTS 157 



nonelectrolytes do not produce any chemotropic effect on roots, 

 while the cations of electrolytes, as a rule, produce a negative 

 chemotropic curvature, and the anions a positive curvature, the 

 effect of salts being in this respect made up of the sum of the 

 effects of their cations and anions. 



Under natural conditions, the role of chemotropism in regard 

 to the 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 the soil appears to have a much greater effect 

 on the growth of roots. 



If a vessel containing the root system of a plant is hermetically 

 sealed except for one small opening, then the roots will grow in 

 the direction of this opening to 

 meet the oxygen diffusing in- 

 ward. This phenomenon has 

 been given the name ^'aerotro- 

 pism," it being but a special 

 type of chemotropism . Humid- 

 ity produces a still greater effect 



on the growth of roots. When Fig. 45.— Hydrotropism of roots. 



water is not equally distributed instead of growing straight downward. 



. , the roots cling to the damp sawdust 



through the soil, the roots may t^after Osterhout). 

 grow in the direction of the 



more humid portion. The bending of roots in the air toward 

 more humid surfaces may be observed (Fig. 45). Such an 

 experiment, however, will be successful only in an atmosphere 

 that is almost saturated with water vapor, for otherwise the 

 roots are apt to dry out. But no hydrotropic curvatures 

 will be secured in an atmosphere completely saturated with 

 water vapor, as in this instance the whole space will be of equal 

 humidity. Like geotropism, hydrotropic sensitivity is localized 

 in the tip of the root. 



An unequal distribution of temperature may also produce 

 bending of both the roots and the organs above ground, showing 

 both positive and negative thermotropism. When the tempera- 

 ture is below the optimum, plants will curve in the direction of 

 the more heated side; when above the optimum, they will bend 

 toward the colder side. Such thermotropic curvatures may be 

 secured with roots growing in moist sawdust in a box whose ends 

 are maintained at different temperatures. 



