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.
Having considered the mechanism and some of the properties and contents of roots, stems, and leaves, the way is now clear to discuss some of the phenomena exhibited by plants as a whole or in a connected way. There is no regular circulation of sap in plants comparable to the blood in animals, nor a constant flow in any one direction, except temporarily. The flow is in many and diverse directions, according to the particular kind of work being conducted and the part of the plant where it is taking place. The movement is a progressive, not a circulating one. Root pressure, taken on the whole, is the most constant or continuous force at work in causing a rise of watery fluid in the plant. The pressure it exerts may be most readily observed in spring, when all the tissues of deciduous trees and shrubs are gorged with water, prior to the expansion of the leaves. The Vine exhibits this pressure in a marked degree, and though it varies within limits, according to the size and vigour of the plant, it has been found to support a weight of nearly 15 lb. to the square inch. This alone enables the sap to rise to the apex of the longest rod, because the force of capillary attraction must also be reckoned with when it is remembered that the cavities of the fibres and vessels of the wood get filled with water and air - a combination that is difficult to move. Trees, shrubs, herbaceous plants, and annuals exhibit this root pressure, but in low-growing plants it is most observable at night and early in the morning, that is, some time after transpiration has ceased and before it commences to take effect again with daylight and a drying atmosphere. Root pressure is of great importance to plants that are making their growth, by keeping their tissues gorged and extended with water, without which growth would be impossible. It is equally important to deciduous trees and shrubs, which require a considerable force to expand their winter buds and urge them into fresh growth. Plants grow more rapidly by night than by day, because root pressure is then exerting its full force. Even if this is not sufficient to raise water to the tops of the tallest trees, a considerable pressure is exerted on the buds by the expansion of the air bubbles in the water of the wood cavities as a result of the rise of temperature in spring.