The radicles, hypocotyls and epicotyls of seedling plants, even before they emerge from the ground, and afterwards the cotyledons, are all continually circumnutating. So it is with the stems, stolons, flowerpeduncles, and leaves of older plants. We may, therefore, infer with a considerable degree of safety that all the growing parts of all plants circumnutate. Although this movement, in its ordinary or unmodified state, appears in some cases to be of service to plants, either directly or indirectly–for instance, the circumnutation of the radicle in penetrating the ground, or that of the arched hypocotyl and epicotyl in breaking through the surface–yet circumnutation is so general, or rather so universal a phenomenon, that we cannot suppose it to have been gained for any special purpose. We must believe that it follows in some unknown way from the manner in which vegetable tissues grow.

We shall now consider the many cases in which circumnutation has been modified for various special purposes; that is, a movement already in progress is temporarily increased in some one direction, and temporarily diminished or quite arrested in other directions. These cases may be divided in two sub-classes; in one of which the modification depends on innate or constitutional causes, and is independent of external conditions, excepting in so far that the proper ones for growth must be present. In the second sub-class the modification depends to a large extent on external agencies, such as the daily alternations of light and darkness, or light alone, temperature, or the attraction of gravity.