As explained in this lesson, at the base of the tree the phloem sap has delivered most of its sugar and is therefore quite watery. Just next to it, the xylem sap is rich in minerals, which were mostly absorbed by the roots. This unbalanced rate of water between phloem and xylem leads to water movement from the first one to the latter one, due to a phenomenon called osmosis. Unsurprisingly, a mirrored water exchange takes place at the top of the tree. Once it has delivered its nutrients and reached the tree’s top, the xylem sap is very watery, and by a similar osmotic movement, water goes from the xylem to the phloem enabling the production of new phloem sap. The water is then circulating in a partially closed circuit, with input from the roots and output from the leave.

As water gets evaporated at the leaves, the pressure inside the xylem rises which creates the so-called follicular aspiration making the xylem sap move upwards. However, this strong increase in pressure might cause dramatic side effect: the localized vaporization of water –this phenomenon is named cavitation- creating gas bubbles called emboli. Embolism can break then the liquid columns formed in xylem tubes, and prevent xylem sap from moving upwards. Trees have therefore evolved different regulatory systems fight against the formation of such emboli. To do so, tree must replace embolized vessels, maintain a highly redundant transport system, or repair embolized conduits, all of these mechanisms are summarized in this scientific publication.

One of the additional characteristic of this circulatory system is its extreme low energy cost. A 100-meter tall tree (so already very high) needs to transport around 100 kilograms of water per day. This work corresponds to 2 Watts. The energy need of smaller plants is therefore much reduced, and sits around 0.1 Watt in average. By comparison, the activity of a human heart at rest is near to 10 watts, knowing that this activity is greatly increased during physical exercises. Trees make the best use of the energy that they have access to. This parsimonious use of energy as well as their ingenious vascular system contribute to their height and their longevity, going sometime over millennium.

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