DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Macrophotograph of the stem of a grape vine Vitis vinifera, cultivar Riesling Sylvaner, showing the effects of root pressure in a cut stem. The stem was cut in early spring when water absoption from the soil due to osmosis was high. In the absence of growing shoots there was nowhere for the water to go, so the root pressure built up until the sap spilled out the cut end of the stem. Sap can bleed at a rate of 2 millilitres a minute for hours or days. This is one of the reasons why pruning takes place in early winter when water absorption has slowed down or ceased. Visible here is the brown heart wood and xylem rays (water conducting vessels).
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