Phloem

From Palaeos

Conducting tissue of vascular plants. Phloem differs from xylem both structurally and functionally. Functionally, phloem transports solutes both up and down the plant, while xylem functions as a one-way transport system for moving water from the roots to the leaves (or equivalent). Phloem carries sugars and other direct or indirect products of photosynthesis to the stem and roots, generally during the day. Phloem also carries ions and soil nutrients up from the roots to the stem and leaves, generally at night. Xylem makes considerable use of the cell walls of dead or enucleated cells. Phloem uses living cells. Xylem uses capillary action and suction, like a straw, to transport water. Phloem uses osmotic pressure to move solutes. A high concentration of solute is loaded inside cells of the phloem at a source, such as a leaf where sugars are produced. This creates a diffusion gradient that draws water into the cells. The resulting pressure causes a flow to occur. If the dissolved sugars or other chemicals carried along with the sugars are removed from the phloem at another place in the tree for use (a sink such as a root or fruit), the decline in concentration of sugar causes water to move out of the phloem cells. Because water is moving in at a source and out at a sink, there is a mass flow of water and substances in the phloem. In trees, phloem is shed annually with the bark.

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