. Cassell's popular gardening. Gardening. 86 CASSELL'S POPULAR GARDEXING. Fie 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 tho circumstances by which it is suiTOunded, by its structure, 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 gra\dtation are counteracted, the roots do not grow downwards. Knight sowed seeds on the outer s
. Cassell's popular gardening. Gardening. 86 CASSELL'S POPULAR GARDEXING. Fie 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 tho circumstances by which it is suiTOunded, by its structure, 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 gra\dtation 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 tho 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 tho 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 tho tendency of gravitation to puU them down, and the tend- ency of centrifugal force to keep thorn horizontal (Fig. 14). The quicker the rotation of the whoel, 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 bo not stagnant or excessive. Then it is well ascertained that the growth in the length of roots occurs chiefly in one particular part of the root, and that the jjortion just above the root-cap or shield before mentioned. How the Radicle Penetrates tlie Soil— Movement of the Hoots.—The conical point cf the root, thus protected, is thi'ust 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 suiDport. And now comes into operation a very marvellous process, only lately b
Size: 2156px × 1159px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade18, booksubjectgardening, bookyear1884
