HOW SAP FLOWS.

Wairarapa Daily Times, Volume VIII, Issue 2463, 29 November 1886, Page 2

HOW SAP FLOWS.

Tho sap of most plants is taken from) the soil by the power of caiisirig osomoais sufficent to raise what ut generally knowa as'" root pressure." In the stem the fluid passed through the vcsaels-'or duclkof the pitted cells or plants which, like ttJ|rpin& have few vessels. Theae are. all minute tubes in which the sap, is' supported by capillary attraction, so that root pressure is generally . considered to be amply sufficient to force the ourre'nt to the top of the highest tree. But its - inotion upwards is induced by the pumping action of the leaves, from which large quantities of water evaporate* The ducts and pitted cells through which the stream flowsi are not entirelyfilled with sap, but includes bubbles of gas with it. Ab water'is withdrawn from the uppermost cells ,by evaporation the air in them expands to occupy 'the additional space, and so exerts pressure than at first. This allows of the water lower down..to be forced upwards, and into them by- the elasticity of the air bubbles in the other cells, the adjustment going on from above downward—the tendency being to equalize the gas pressure throughout tho entire plant', While evaporation continues this equilibrium i# never roached; when it stops the balance may. be affected, and the sap remaiu qtwei, supported by root pressure and cawbrity. unless somo disturbing element is introduced.