Why Plants Need Water
WATER AND TRANSPIRATION
The need of a plant for water is constant. Water forms 90 per cent or more of the substance of most plants. It is the solvent that carries nutrients from the soil through the roots and stems to the leaves, and foods from the leaves to every other part of the plant. It takes part in all the living plant processes and provides hydrogen and some of the oxygen for the building up of carbohydrates and other plant foods.
A plant invariably tends to take up more water than it can use. The excess is eliminated by the leaves. Their thin, flat structure and interior arrangement of loose cells with many air spaces between makes this possible. The surplus water oozes through the cell walls and vapourizes in the air space surrounding them. A little goes through the skin, but most of it moves out through the stomata.
This giving off, or evaporation of water by the leaves of plants, is called transpiration. A green plant in active growth may absorb and transpire anything from about one quarter to twice its own weight of water in a day. It has been estimated that an average-potato plant can use 25 to 35 gallons of water in a growing season, while a large tree may transpire water at the rate of 10 to 15 gallons an hour. Almost all the water comes from the soil, although leaves can absorb a little from rain, dew or artificial watering.
The water given off escapes as an invisible vapour into the atmosphere; but in the cool upper layers of the air it condenses, forms cloud and returns to the earth as rain. In this way, the earth’s vegetation helps to maintain the humidity of its climates. It is a cycle in which trees are especially important; their roots search out water in the deep layers of the soil, and because of their enormous area of leaf surface they give off more water vapour than the smaller plants. They can almost be said to create, and attract, their own rain.
The raising of water in the tree, through the microscopically narrow tubes of the xylcm to the topmost leaves, is a tremendous feat of hydraulics. The manner in which it is accomplished is not fully understood. It is believed that transpiration sets up an osmotic pressure in the leaf cells losing water vapour to the air spaces surrounding them. This causes water to flow into them from neighbouring cells, and these in turn draw water from the xylem in near-by leaf veins.
In the xylem cell tubes there are extremely narrow capillary columns of water which are continuous, extending back through leaf veins, stem and roots to the tiny root hairs. Drawn by the osmotic pressure in the leaves, the water column moves up, almost as a solid, while at the lower end a pressure is created in the root hairs to absorb more water from the soil. It is this process which explains the occasional wilting of cut flowers when they are placed in water. The end of the stem has been cut in air and has drawn some into the xylem tubes; as a consequence, an air lock forms and it loses its power to pass on moisture. The end of some stalks, especially those which are hollow, as they are in delphiniums, should always be cut under water if flowers are to remain fresh.
The transpiration stream can move at a speed of about 3 in. a minute in day-time, but the rate of transpiration is not constant. It increases as temperatures rise and in dry or windy conditions, and slackens in cool, humid or rainy weather. It is greatly reduced at night when photosynthesis stops and the stomata close.
When the root hairs are unable to find and absorbquickly enough to maintain transpiration, growing points, leaves and stems droop, and the plant wilts. Unless the water position is restored, the plant will gradually die as its cells dry out.