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.
If a Vine rod is cut into the wood in spring, before the expansion of the leaves, it will bleed strongly for many days, and may even die or become so weak as to be useless. Its tissues are gorged with watery fluid containing a considerable amount of liquid plant food, dissolved by the water from cells where it was stored. Root pressure is very strong at this period, and the vessels of the wood of the Vine are very large. Much of the stored food is thus lost, as well as the pressure of sap necessary to start the dormant buds into fresh growth. If a Vine is cut in June or July, when in full leaf, it will not bleed, because the vessels are then filled with air, as a result of the transpiration current. Inarching of Vines should not be attempted till they are fairly into leaf, for these reasons. If pruning is accomplished in autumn, before the wood is perfectly ripened, Vines often bleed in spring when root pressure becomes strong. This phenomenon of bleeding, when cut in spring, may be observed in various other plants, including the Birch and Maple. The former will bleed at a considerable height from the ground (12 to 15 ft.), though most strongly near the base of the trunk. Sugar is extracted from the sap of the Sugar Maple, and Rubber from the sap of various Rubber Trees. [J. f.]