To help towards a clear understanding of both tendencies, Goethe describes an exercise which is characteristic of his way of schooling himself in what he called exact sensorial fantasy. He first looks out for a phenomenon in which the 'secret' of the spiral tendency is made 'open'. This he finds in such a plant as the convolvulus; in this kind of plant the vertical tendency is lacking, and the spiral principle comes obviously into outer view. Accordingly, the convolvulus requires an external support, around which it can wind itself. Goethe now suggests that after looking at a convolvulus as it grows upwards around its support, one should first make this clearly present to one's inner eye, and then again picture the plant's growth without the vertical support, allowing instead the upward-growing plant inwardly to produce a vertical support for itself. By way of inward re-creation (which the reader should not fail to carry out himself) Goethe attained a clear experience of how, in all those plants which in growing upwards produce their leaves spiral-wise around the stem, the vertical and spiral tendencies work together.