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 slender, 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 describes a somewhat corkscrew-like spiral. This movement he called circumnutation, and found that essentially similar movements (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 inkdots 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 vertical 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 surface 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 circumnutation 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 contributions to our knowledge of the root-tip that have ever been made.