THE ROOT-TIP. 33 



the results could hardly fail to be disastrous : for, in the first place, 

 the resistance of the earth would soon cause a strong curvature ; 

 and, in the second place, the tender apex would be injured by- 

 being thus forced against the earth. As it is, the tip penetrates 

 the earth, not like a nail driven by a force behind, but like a slen- 

 der, tapering cone whose point insinuates itself between the earth- 

 particles and then by growth in thickness wedges them apart. 

 Experiment has shown that a root in its longitudinal growth 

 exerts but very little force ; in the bean, for example, there is 

 scarcely force enough to raise a quarter of a pound. The force of 

 transverse growth, on the other hand, is considerable equal in 

 the bean to the raising of over eight pounds.* 



It was first demonstrated by Darwin that the elongation of the 

 root takes place in such a way that the apex, instead of going 

 straight forward, bends to all sides in succession and thus de- 

 scribes a somewhat corkscrew-like spiral. This movement he 

 called circumnutation, and found that essentially similar move- 

 ments (some of which had been before observed) were exhibited 

 by all growing stems and leaves, and not infrequently after 

 growth had ceased. In the case of the root, the movement may 

 be rendered apparent in either of two ways. One method is to 

 take a seedling growing in moist air, and magnify the movement 

 of the root-tip by attaching to the bending portion a very slender 

 filament of glass several inches in length, and then, on a sheet of 

 glass kept perpendicular to the axis of the root, record by ink- 

 dots the different points to which the filament is from time to 

 time directed. Upon connecting the dots made at short intervals 

 through a period of several hours, a result is obtained somewhat 

 like that shown in Fig. 2. The other method is to allow the ver- 

 tical root of a seedling to grow downward against the smoked 

 surface of a piece of glass which is held oblique to the axis. If 

 the conditions are favorable, the tip will be found to rub the sur- 

 face and leave a serpentine tracing similar to those given in Fig. 

 3. That the course of the tip had been spiral and not zigzag 

 was shown in Darwin's experiments by alternating regions of 

 greater and less rubbing, and in some cases by transverse ridges 

 of soot. Since these experiments can not be performed with the 

 root imbedded in compact earth, we can not say how far circum- 

 nutation may take place in ordinary soil, but undoubtedly the 

 tendency to circumnutate is ever present, and whenever there is 

 favorable opportunity for its exercise the spiral movement must 

 materially assist the tip in making its way along the line of least 



* For the details of this experiment, as of others to be mentioned later, the reader is 

 referred to Darwin's Power of Movement in Plants, which contains the most valuable con- 

 tributions to our knowledge of the root-tip that have ever been made. 

 vol. xxxviii. 3 



