54 Commercial Gardening 



When the temporary reserve in leaves consists of starcn, it is first con- 

 verted 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 Monocoty- 

 ledons 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. 



Water Plants. As these contain little (or no) woody matter in their 

 tissues the rise of sap is of a feeble character, but when wholly submerged 

 there is no transpiration current at all. When the roots are in soil, food 

 would be brought in that way, while the oxygen for breathing purposes 

 and the carbon dioxide absorbed by the leaves are taken directly from 

 the surrounding water. The leaves and stems have no cuticle, so that 

 each cell comes in direct contact with water holding food in solution. 

 Floating plants get all their food directly from the water, except the 

 carbon dioxide of the air available to the exposed leaves. 



Sap in Winter. When the leaves fall in autumn, transpiration ceases. 

 Root pressure continues till all the tissues get filled with water and turgid, 

 including the cavities of the wood fibres and vessels, which also contain 

 air. The gradual falling of the temperature also makes the roots less 

 active, though not entirely dormant. A considerable number of plants 

 bloom in winter and require a modicum of water. Evergreen trees, shrubs 

 and herbs, which retain their leaves, must have a certain supply of liquid 

 to keep them turgid and alive. The roots themselves keep extending for 

 an unknown length of time after the fall of the leaf, except when the 

 ground is frozen; and what is used up in these various ways must be made 

 good by the absorbent hairs and superficial cells of the younger roots. It 

 is natural and necessary that the roots of plants belonging to temperate 

 climates should have an adequate supply of water even in winter. This 

 explains bud-dropping in Peaches that have been allowed to get over dry 

 at the roots in winter till the buds perish, as the extremities of the trees 



