This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
When the temporary reserve in leaves consists of starcn, it is first converted into liquid sugar or glucose, and can then be conveyed to the growing points of stems and branches, with the young, undeveloped leaves upon them, to flowers, fruits, and seeds. This necessarily means minor currents of slow motion, with usually short distances to travel. Towards the end of the season, when growth is more or less completed, much of the food prepared in the leaves must be carried away and stored in the trunks of trees, in bulbs, corms, tubers, tuberous roots, taproots, and other parts of plants according to the kind. This implies downward currents of water, containing the food materials dissolved in them, and these must also be slow movements. In the case of trees and many other plants it is well known that a considerable number of new roots are made in autumn. This is due to the warmth of the ground and the autumn rains, as well as to the existence of a plentiful supply of ready-made food. The rain softens the previously dry and hard earth, and thus enables the roots to penetrate it and extend their system. In trees this food must often travel considerable distances, but rapid transport is favoured by the presence of sieve tubes or continuous vessels in the hard bast, situated in the inner and younger layers of bark. In Palms and other Monocotyledons this hard bast is situated in the isolated fibro-vascular bundles, as there is no bark in these cases. Storage may take place in cells that contain no protoplasm.