Terry Chomey, age 13i of Allentown, PA, for his question:
What is Osmosis?
How does water flow upwards from the roots to a tree top? How do fragile fresh petals stay firm as parchment? How does a little root push a crack through a stoney rock? We have all been amazed and have wondered at these things. Well, Nature uses a very simple trick to power all those jobs. It is osmosis. And you can do a simple experiment to watch it work.
You will need a pan of water; a little sugar and water solution, a hollow glass tube and a piece of sausage skin. The sausage skin is a membrane, a fabric used in both plant and animal tissues, Use it to seal one end of the tube and pour in the sugar solution. Place the tube upright with the tightly sealed end in the pan of water • and wait.
The sugar solution in the tube will rise. Plain water seeps through the membrane from below. This is all there is to the trick of osmosis. Water; or a weak solution pushes through a membrane and adds to a stronger solution. The trip may be uphill or down dale,
Ground water contains chemicals dissolved from the soil. But tree sap is a far richer solution of dissolved sugars. The two are separated by tender root membranes. Water in the soil must push through the membranes to join the sap in the plant.
Meantime the plant is losing moisture through its leaves, A big oak loses maybe 100 gallons of water on a summer's day. This makes space for the sap to rise up and fill. Water molecules attract each other and tend to cling together. One follows another in a steady flow uphill from roots to leaves. Simple, but mighty powerful.
Osmosis creates pressure and energy, Not all plants grow stiff, woody material to hold them up, The pressure of osmosis, osmotic pressure, works to keep leavgs, roots, petals and tender stems proud and firm. Cut flowers droop when the flow of moisture is stopped. Put their feet into water and they soon become firm again.
Buds and unfolding flowers use energy from osmosis. Watch the green cradle break around a pretty poppy bud. The petals are crumpled like wads of tissue paper. After a few hours of sunshine and breezes they are smooth as bright silk, The wrinkled petals have been ironed out by osmotic pressure, The same energy helps the leaves to open and the young buds to unfurl: It helps lush fruit to burst its skin and free the captive seeds within„
An adventurous root is strong with this osmotic pressure, Its job is to seek moisture in the soil: It pushes down, using its strength to move the earth and make way. A root no bigger than your finger has hundreds of pounds of osmotic pressure with which to shove and dig. No wonder tree roots can crack open weak spots in stoney rocks.