Please take a moment to read this wonderful evocation of the Rising of the Sap, without which we would have no trees or plants!
The Rising of the Sap – by James Le Fanu
The joys of spring are irresistible and this year more so than ever. After three dreary months of leaden skies and icy winds nature’s dazzling reawakening came in a rush as, almost over night, the cherry blossom blossomed, the tulips and magnolia ‘burst forth’ and the buds of the horsechestnut unfolded like so many small iridescent stars. One day all seemed grey and monochrome, the next resonant with pinks, whites, yellows and vivid greens. “Can there be anything so wonderful as this meeting of death and life” enquired Professor J Arthur Thomson in his classic book The Biology of the Seasons. “The power of trees and plants to transmute by secret alchemy the elements of water, soil and air into the bread and wine of life”.
Spring is no ordinary event imaginatively celebrated by our ancestors, supposedly less sophisticated than ourselves, in myth and folklore: Persephone’s annual return from the Underworld, The Sleeping Beauty woken from her prolonged slumbers by a Prince’s kiss. But this resurgence of life is indeed magical – exemplified most dramatically by ‘The Rising of the Sap’ from deep within the soil to the dizzying heights of the topmost branches of the oak, elm and horsechestnut.
Consider for a moment the epic passage of a drop of that life sustaining mix of water and its precious nutrients, iron, chlorides, phosphates and nitrogen. That journey starts with its absorption into the jellified substance of the roots from where it is propelled forward into the network of narrow ‘drainpipes’, the xylem, a few microns in diameter at the base of the tree. At this point another much more potent force takes over sucking the drop of sap upwards into the veins of the leaves that rob it of its nutrients before finally shoving it outwards into the fresh air through the minute holes or stomata on their undersurface.
The pressure required to impel that column of fluid a hundred foot upwards against the pull of gravity and the resistance of the tissues lining ‘the xylem’ are truly astonishing. The technical terminology may not be very familiar but put simply we experience an atmospheric pressure of roughly one hundred thousand pascals, or 0.1 mega (millions) pascals( MPa). By comparison the pressure required to inflate a car tyre is twice as great at around 0.2 MPa. ‘The rising of the sap’ requires pressures fifteen times greater still at 3.0 MPa more than sufficient to crush the life out of any creature. It is impossible to conceive of a biological pump – such as the human heart – that might be capable of generating such pressures. But the horsechestnut, of course, has no pump rather the rising of the sap is a direct (if extraordinary) consequence of two unique physical properties of water itself.
The first is the well known phenomenon of osmosis on which so many biological functions depend where water moves across the semipermeable membrane of the cell wall to equalise the concentration of solutes on either side – readily illustrated by a simple experiment involving a sausage skin filled with a strong sugary solution. Tie the sausage skin tightly at one end, attach a glass tube to the other and immerse in a bowl of water. Before long the water will move inwards causing the sausage skin to bulge and a column of fluid will be noted rising up the glass tube. And so too the roots of the horsechestnut whose jellified interior absorbs fluid from the soil and propels it upwards into the xylem’s drainpipes.
But the osmotic pull of the roots, it emerged, could only make a modest contribution as demonstrated by another experiment which involved cutting a fifty foot oak tree off at its base and placing it upright in a bath of red dye – that was duly sucked upwards to infiltrate and discolour its leaves. In 1894 the Professor of Botany at Trinity College Dublin Henry Horatio Dixon attributed the sap’s continued upward movement to the tension exerted by its evaporation through the stomata of the leaves – in a manner not dissimilar to sucking liquid through a straw where the powerful chemical bonds binding its molecules together ‘pull up’ those behind it.
The practicalities of the continuous upward flow of a hundred foot column of sap are inevitably much more complex – indeed a dissenting group of botanists maintain that Professor Dixon’s CohesionTension theory (as it is known) is by itself inadequate for the task.
But no matter.It is enough the search for an explanation of the rising of the sap should draw attention to the natural miracle of what it entails: how to shift the nutrients of the soil into the canopy of the horsechestnut so as to transform, in a few short weeks, those star like budding leaves into its magnificent foliage – without there being any powerful mechanical ‘engine’ to do the shifting. That is no ordinary achievement.