Science Sittings

New Zealand Tablet, 17 July 1913, Page 49

Science Sittings

By 'Volt.'

The New Theory. The fact that water, absorbed from the soil by the millions of fine hairs covering the fibrous ramifications of the roots, ascends to the top of lofty trees, carrying raw food material to the leaves, which give off the excess of moisture in the form of water-vapor, is ■ one'of those familiar phenomena which at first sight appear to be perfectly simple. In reality, writes Professor F. Cavers, D.Sc, F.L.S., in the University Correspondent, the ascent of water in trees presents some of the most difficult problems in the physiology of plants, and has for many years been the subject of controversy, numerous mutually contradictory theories having been successively set up and demolished, with the result that the question cannot even now be said to have been completed solved. Though the question is not generally regarded as finally settled yet, it would appear that the solution is to be found in the purely physical properties of the wood vessels and of the water stream itself, without relying upon any special vital activity on the part of the cells of the plant. The new physical theory of sap ascent is based upon the fact that a column of water possesses great cohesive strength, and can sustain a great tensile stress other words, that under certain conditions (corresponding exactly with those present in the wood vessels) a water column can transmit a pull just like a rigid solid, and that the water in the wood of high trees hangs there by virtue of its cohesive strength. According to this theory the flow of water is due to the tension set up in the leaves as they lose water by evaporation.

The Problem of Aeronautics.

An interesting problem 'of aeronautics— far man should imitate the flight of birds and insects in his efforts to conquer the airwas discussed lately by Mr. Horace Darwin, when he delivered the first Wilbur Wright memorial lecture before the Aeronautical Society at the Royal United Service Institution. He described many of the wonderful instruments of measure which have been evolved as the science of aviation has progressedinstruments to measure speed during flight and during ascent or descent, and the ' yaw-meter,' which shows the direction of the wind and measures sideslips. With regard to the resemblances and the differences between the flight of men and the flight of birds, he said: These resemblances are remarkable, but there are great differences. The Wright brothers found no biplane bird to copy, and did not flap their wings. No flying animal uses a continuously rotating propeller to drive him forward on soaring wings, and it is perhaps hardly too much to say that if birds only knew how, they would now copy the Wright Brothers. Muscular action and the circulation of the blood, however, put supreme difficulties .in the way of the development of the continuous rotation of a part of an animal. Cranks and connecting rods, as well as rotating valves to allow the circulation of the blood, would be required. The development of the power of flight in birds has been so slow that we cannot realise the time taken, or form the roughest estimate in years; but the perfection of these adaptations and the beauty of their skill, strength, and movement must strike anyone who has ever watched their flight. Some less advanced animals have only learnt to glide, and are now in the same stage, of development as the Wrights were a few years ago. Perhaps these gliders developed more slowly or perhaps only began to learn the art many ages after birds had learnt to fly. A few plants also have developed wings to their seeds, so that they can glide away to more suitable places for germination and growth.