. Cassell's popular gardening. Gardening. 86 CASSELL'S POPTJLAE GARDENING. problem not yet completely solved; how it does so is more apparent, and the results are of course obvious. The reasons for its descent are to be found in the circumstances by which it is surrounded, by its structm-e, and by its mode of growth. Gravi- tation has been shown by Thomas Andrew Knight and many subsequent observers to influence the downward course of roots, or rather, it has been proved that where the effects of gravitation are counteracted, the roots do not grow downwards. Knight sowed seeds on the outer side
. Cassell's popular gardening. Gardening. 86 CASSELL'S POPTJLAE GARDENING. problem not yet completely solved; how it does so is more apparent, and the results are of course obvious. The reasons for its descent are to be found in the circumstances by which it is surrounded, by its structm-e, and by its mode of growth. Gravi- tation has been shown by Thomas Andrew Knight and many subsequent observers to influence the downward course of roots, or rather, it has been proved that where the effects of gravitation are counteracted, the roots do not grow downwards. Knight sowed seeds on the outer side of a wheel kept rotating in a vertical direction. By these means the force of gravitation was overcome or replaced by centrifugal force, or the tendency to separate or fly off from a revolving body. Under these circumstances the roots always turn outwards, away from the centre of the wheel (Fig. 13). When the seeds are sown on a horizontally moving wheel, the roots assume an intermediate position between horizontal and vertical, as the result of a compromise between ' the tendency of gravitation to pull them down, and the tend- ency of centrifugal force to keep them horizontal (Fig. 14). The quicker the rotation of the wheel, .the more horizontal the direction of the root. The presence of air is es- sential to the roots, and also the existence of moisture, provided that it be not - stagnant or excessive. Then it is well ascertained that the growth in the leng-th of roots occurs chiefly in one particular part of the root, and that the portion just above the root-cap or shield before mentioned. How the Radicle Penetrates the Soil— Movement of the Roots.—The conical point of the root, thus protected, is thrust forward, or rather downward, into the soil by the lengthening that takes place just above it, the body of the seed and the superincumbent soil acting as the " fulcrum," or point of support. And now comes into operation a very marvellous process, only lately br
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Keywords: ., bookcentury1800, bookdecade18, booksubjectgardening, bookyear1884
