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Feeding in Green Plants

Walnut tree, Bristol, UK © Shirley Burchill

All living organisms must feed so that they can have energy. Living organisms must also feed so that they can grow and remain healthy. Plants are no exception to this. They need to feed but they do not feed in the same way as animals. They do not have a mouth or a digestive system and they do not move much to find and capture their food.

The biggest organisms on Earth are plants. The giant redwood tree of California and the eucalyptus tree from Australia grow to 100 metres in height. These giants may have a mass of 600 tonnes but they grow from tiny seeds of just a few grammes.

Facts iconJohnny's Fact File No.22
A single wheat plant has a total of 70 km of roots and a single rye plant has a total of 80 km of roots!
If a single
maize plant is allowed to grow away from any other plants its roots would occupy 8 dm3 of soil.

Plants are made of three things. Water is the most important part. Often more than 90% of the plant is made of water. Water, however, is a liquid but plants are very solid, especially when they are made of wood.

If we dry a plant out by removing all of its water content, we can find out what is left over. Two things are found in the solid parts of plants : minerals and organic matter. We can separate these two parts if we burn the dried plant until only the ashes are left. If we analyse these ashes we find that they are made of the minerals that the plant needs to grow. The part that burns away is called organic matter.

The Passage of Water through a Plant

Seedling showing root structure © Shirley Burchill

Seedling showing root hairs © Paul Billiet

It is true that plants need a lot of water to stay healthy and plants are mostly made of water. A pumpkin, for example, is 94% water. If we do not water plants they will wilt and eventually they die. Most of the water we give to a plant does not stay in its body. The water passes through the plant and evaporates into the air.

Plants absorb most of their water through their roots. The roots of a plant can be very extensive, as much as 50 kilometres of roots can be found in one square metre of soil. A single wheat plant has a total of 70 km of roots and a single rye plant has a total of 80 km!

If you observe the ends of the roots carefully you will see that they look furry. This 'fur' is made of the root hairs and this is where most of the water is absorbed.

Facts iconJohnny's Fact File No.23
1 hectare (10000 m2 ) of wheat consists of 2,5 million individual plants. These plants absorb 4,5 million dm3 of water in their ten months of life.
 
eucalyptus tree transpires 7,5 g of water from 1 dm3 of leaf surface in one hour.
birch tree with 250000 leaves loses 360 dm3 of water per day.
oak tree can transpire about 500dm3 of water in a day

From the roots the water travels up the stem in very thin tubes or vessels. It moves towards the leaves. The water travelling through a plant and the substances dissolved in it is called sap. Eventually the water arrives in the leaf where it escapes into the air as water vapour.

The evaporation of water from the leaves of plants is called transpiration. The water vapour passes through microscopic pores in the leaf surface called stomata (singular: stoma).

Stomata as seen under high power © Paul Billiet

The stomata can open and close to control how much water the plant loses. Even so a large plant with a lot of leaves may transpire a large amount of water every day.

Even more water will be lost if the weather is hot, dry and windy. All this water which is lost must be replaced by the water absorbed by the roots.

Plants without Roots

Moss © Shirley Burchill

Mosses are simple plants which do not have roots to absorb water. They do not have vessels in their stems to transport water either. Mosses are so small and delicate that they can absorb all the water they need through their leaves. You will usually fin mosses growing where they will receive a lot of water as rain or mist.

When the weather is dry for a long time the moss plants look brown and dead but as soon as they take in water they quickly come back to life.

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