36 



GENERAL BOTANY 



moving his head from side to side, or circumnutating, he will feel 

 any stone or other obstacle, as well as any difference in the hardness 

 of the soil, and he will turn from that side ; if the earth is damper 

 on one than on the other side, he will turn thitherward as a better 

 hunting-ground. Nevertheless, after each interruption, guided by 

 the sense of gravity, he will be able to recover his downward course 

 and to burrow to a greater depth. 



Darwin's contributions to our knowledge of the sensitiveness 

 of the root tip have been confirmed by later researches and have 



done much towards 

 elucidating our ideas 

 concerning the nature 

 of stimulus and re- 

 sponse as applied to 

 plants. Fig. 20 illus- 

 trates the principle of 

 some ingenious experi- 

 ments by Czapek, who 

 confirmed Darwin's gen- 

 eral idea that the sen- 

 sitiveness of the root 

 to gravity is located 

 mainly in the root tip. 

 Czapek forced roots to 

 grow into bent tubes, 

 as illustrated in the 

 figure, so that the last 

 millimeter of the root was at right angles to the body of the root. 

 Such roots, when placed in the position indicated in Fig. 20, a, 

 produced a curvature like that in b. Root tips placed in a 

 position like that of c, however, failed to curve in response 

 to gravity, as indicated in <#, since the sensitive tip was in the 

 normal progeotropic position. When, therefore, the terminal 

 millimeter of the root tip is placed in a position of stimulation 

 (Fig. 20, a), curvature of the root occurs ; but when the terminal 

 millimeter is placed vertically (<?), no curvature results. We may 

 conclude, therefore, with Czapek, that the last millimeter of the 



FIG. 20. Drawings illustrating experiments by 



Czapek, designed to locate the sensitive zone of 



the root to gravity acting as a stimulus 



The roots of the bean were grown into glass slippers 

 and then placed so as to expose the last few milli- 

 meters of the tip to gravity at different angles from 

 the vertical. , position of the root tips at the begin- 

 ning of the expei-iment; 6, position assumed by the 

 above root tips twenty hours later - c, root tips placed 

 vertically at the beginning of the experiment : d, posi- 

 tion assumed by the same roots eighteen hours later. 

 Further discussion in the text. How do you explain 

 the difference in the position assumed by roots in b 

 and d ? After Czapek 



