The body of every organism (plant or animal) is made up of cells. A large organism has millions and millions of cells in its body. In order that the organism may be able to maintain its life and survive, all its cells must be supplied with essential substances like food, oxygen, water, etc. So, some arrangement is required inside an organism which can carry the essential substances to all its parts so that they reach each and every cell of its body.

In everyday language, ‘transport’ means ‘to carry things from one place to another’. In biology, transport is a life process in which a substance absorbed (or made) in one part of the body of an organism is carried to other parts of its body. Large organisms (large plants and animals) need transport systems in their bodies to supply all their cells with food, oxygen, water, and other materials. In fact, special tissues and organs are needed for the transport of substances in plants and animals because these tissues and organs can pick up the essential substances like food, oxygen, water, etc., at one end of their body and carry them to all other parts.

We will now study the transport system of plants and of human beings, and describe the parts which make up these systems. In other words, we will learn how plants and animals carry substances from one part of their body to another.

Transport in Plants :

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Transport system in plants is less elaborate than in animals (including human beings). Plants are less active, so their cells do not need to be supplied with materials so quickly. Also, due to the branching shape of a plant, all the cells of a plant can get oxygen for respiration and carbon dioxide for photosynthesis directly from the air by diffusion. So, the only substances which are to be supplied to a plant through a transport system are water and minerals (which they can’t get from the air).

Another job of the transport system of plants is to transport food prepared in the leaves to the various parts of the plant like stems, roots, etc. The plants have two types of conducting tissues called xylem and phloem. Xylem tissues carry water and minerals whereas phloem tissues carry the food prepared by the plants (see Figure 71). We can now say that:

The plants have two transport systems:

1. Xylem which carries water and minerals, and

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2.Phloem which carries the food materials which the plant makes (Phloem also carries the hormones made by the plants in their root and shoot tips).

The transport of materials in a plant can be divided into two parts:

(i) Transport of water and minerals in the plant, and

(ii) Transport of food and other substances (like hormones) in the plant.

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We will now discuss both the parts of the transport system in plants in detail, one by one. Let us stall with the transport of water and minerals in the plants.

Transport of Water and Minerals:

Plants require water for making food by photosynthesis. Plants also need mineral salts for various purposes (such as making proteins, etc.). Water and minerals are absorbed from the soil by the roots of the plant and transported to the various parts of the plant like stem, leaves and flowers. In the leaves, water is used in making food by photosynthesis. The water and minerals dissolved in it move from the roots of the plant to its leaves through the two kinds of elements of the xylem tissue called xylem vessels and tracheids. Xylem vessels and tracheids are both non-living conducting tissues which have thick walls. Let us discuss xylem vessels and tracheids in a little more detail.

1. Xylem Vessels :

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The xylem vessel is a non-living, long tube which runs like a drainpipe through the plant. A xylem vessel is made of many hollow, dead cells (called vessel elements), joined end to end. The end walls of the cells have broken down so a long, open tube is formed. Xylem vessels run from the root! of the plant right up through the stem and reach the leaves. The xylem vessels branch into every leaf of the plant.

Xylem vessels do not contain the cytoplasm or nuclei. The walls of xylem vessels are made of cellulose and lignin. Lignin is a very hard and strong substance, so xylem vessels also provide strength to the stem and help to keep the plant upright. Wood is made almost entirely of lignified xylem vessels. Xylem vessels have pits in their thick cell walls. Pits are not open pores. Pits are the thin areas of the cell wall where no lignin has been deposited. Pits have unthickened cellulose cell wall. In flowering plants, either only xylem vessels transport water or both xylem vessels and tracheids transport water.

2. Tracheids :

Tracheids are long, thin, Spindle shaped cells with pits in their thick cell walls. Water flows from one tracheid to another through pits. Tracheids are dead cells with lignified walls but they do not have open ends, so they do not form vessels. They are elongated cells with tapering ends.

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Even though their ends are not Tracheids completely open, tracheids have pits in their walls, so water can pass from one tracheid to another through these pits. Although all the plants have tracheids, they are the only water conducting tissue in non-flowering plants.

Before we describe the mechanism of the transport of water from the roots of a plant to the leaves, we should know the meanings of the terms epidermis, endodermis, and root cortex and root xylem. The outer layer of cells in the root is called epidermis. Epidermis is only one cell thick.

The layer of cells around the vascular tissues (xylem and phloem) in the root is called endodermis (It is the innermost layer of cortex). The part of root between the epidermis and endodermis is called root cortex. And the xylem tissue present in roots is called root xylem. Please note that in a root, the root hair are at its outer edge but the root xylem vessels (which carry water to the other parts of plant) are at the centre of the root. And in-between the electron root hair and root xylem are epidermis, root cortex and endodermis. So, before water absorbed by root hair from the soil reaches the root xylem, it has to pass through the epidermis, root cortex and endodermis.

Another point to be noted is that the minerals needed by the plants are taken up by the plants in inorganic form such as nitrates and phosphates. These minerals are present in the soil. The minerals present in soil dissolve in water to form an aqueous solution. So, when water is transported by the root of the plant to its leaves, then the minerals dissolved in it also get transported along with it.