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.
As above stated, this current is set up by the action of the leaves, and by some has been described as a "transpiratory pull". The effect it has towards the base of the stem is that of "suction". It acts only during the day, and, in the case of deciduous trees, only comes into play when the leaves are expanded and the air pores or stomata are sufficiently developed to commence work. When transpiration has been at work for a time the cavities of the wood fibres and vessels get drained of their liquid contents and filled with air, root pressure is subjected to a negative pressure as a result, and the up current of transpiration is dominant. When pot plants and those in the ground are well supplied with water, and their leaves do not flag, it is clear that the great volume of water being given off by the leaves must be coming directly from the roots, and that the absorbent activity of the latter is equal to the demand made upon them. It has been calculated that hundreds of pounds of water are given off by the leaves of trees on a hot day (see p. 120). The water of transpiration is the most rapid current of watery fluid known in plants, and is most characteristic of woody plants; it is more feeble in herbaceous plants with a less-developed vascular system; and non-existent in cellular plants. Since the vessels of the wood are filled with air, the water must necessarily travel in the walls of the wood, so that there is a continuous passage or highway for it from the longest or remotest root fibres to the tips of the leaves. In trees, like the Oak, Ash, Lime, Apple, Cherry, and all other Dicotyledons, there is one main current through the trunk. This flow is chiefly through the younger or sap wood, also known as alburnum, less feebly through the heart wood or duramen, owing to obstructions caused by age. The pith and bark may be removed without causing any diminution in the rise of the sap. In Monocotyledons, like Palms, there is not one main current, but hundreds of small ones in the trunk of a good-sized tree. The transpiration current rises in the small, isolated fibro-vascular bundles distributed through the ground tissue.
In the case of the two great causes of the movements of water in plants just discussed, it must be presumed that the temperature is adequate, since they take place under natural conditions. When Vines, Peaches, Figs, Melons, Cucumbers, and Tomatoes are being forced, out of their natural season, the necessary temperature best suited to each subject must be supplied artificially. Feeding, watering, and the amount of atmospheric moisture have to be controlled likewise by the cultivator, if the best produce is to be secured. Light is liable to be deficient in winter, and full advantage must be taken of it, considering how vital it is to growing plants, from the start till the fruit is matured. This requires properly constructed houses with the glass kept clean. Ventilation, during the middle of the day, at least, is beneficial in promoting transpiration, drying the atmosphere, keeping the functions of the leaves in healthy condition, and hardening the tissues of stems and leaves by the proper thickening of their cell walls, thereby preventing them from becoming unduly " drawn ". The houses can be closed early in the afternoon, thereby conserving the sun heat, if any, for that is more favourable to growth than artificial heat. The foliage may be syringed, if the nature of the weather for the time being warrants or requires it; transpiration will be immediately checked and root pressure will soon begin to exercise its powerful effect upon growth, so that no time is lost by giving timely and judicious ventilation.