The plants take in water (containing dissolved minerals) from the soil through their roots. This water (containing minerals) called xylem sap is carried by the xylem vessels to all the parts of the plant. This happens as follows: The roots of a plant have hair called root hairs. The function of root hairs is to absorb water and minerals from the soil.
The root hairs are directly in contact with the film of water in-between the soil particles. Water (and dissolved minerals) gets into the root hairs by the process of diffusion. The water and minerals absorbed by the root hair from the soil pass from cell to cell by osmosis through the epidermis, root cortex, endodermis and reach the root xylem.
The xylem vessels of the root of the plant are connected to the xylem vessels of its stem. So, the water (containing dissolved minerals) enters from the root xylem vessels into stem xylem vessels. The xylem vessels of the stem branch into the leaves of the plants. So, the water and minerals carried by the xylem vessels in the stem reach the leaves through the branched xylem vessels which enter from the petiole (stalk of the leaf) into each and every part of the leaf.
In this way, the water and minerals from the soil reach through the root and stem to the leaves of the plant. Only about 1 to 2 per cent of the water absorbed by the plant is used up by the plant in photosynthesis and other metabolic activities. The rest of water is lost as water vapour to the air through transpiration.
Water moves up into xylem vessels in the same way that a cold drink moves up a straw when we suck at the upper end of the straw. Now, when we suck a straw, we are reducing the pressure at the top of the straw. The cold drink at the bottom of the straw is at a higher pressure (which is atmospheric pressure), so the cold drink flows up the straw into our mouth.
The same thing happens with the water in the xylem vessels. The pressure at the top of the xylem vessels (in the leaves) is lowered whereas the pressure at bottom of the xylem vessels remains high. Due to this water flows up the xylem a vessel into the leaves. An important question now arises: How is the pressure at the top of the xylem vessels reduced? The pressure at the top of xylem vessels in a plant is reduced due to transpiration. This is discussed below:
The evaporation of water from the leaves of a plant is called transpiration. The leaves of a plant have tiny pores on their surface which are called stomata. A lot of water from the leaves keeps on evaporating into the air through the stomata.
This loss of water (as water vapour) from the leaves of a plant is called transpiration. Since the cells of the leaf are losing water by transpiration, so water from the xylem vessel in the leaf will travel to the cells by osmosis to make up this loss of water.
Thus, water is constantly being taken away from the top of the xylem vessels in the leaves to supply it to the cells in the leaves, reduces the effective pressure at the top of the xylem vessels, so that water flows up into them (from the soil).
Thus, the continuous evaporation of water (or transpiration) from the cells of a leaf creates a kind of suction which pulls up water through the xylem vessels. In this way, the process of transpiration helps in the upward movement of water (and dissolved minerals) from the roots to the leaves through the stem.
Leaves make food by the process of photosynthesis. The food made by leaves is in the form of simple sugar (glucose) substances called plant hormones are made in the tips of roots and shoots. Now, every part of the plant needs food. So, food made in the leaves of a plant has to be transported (or carried) to all the parts of the plant like branches, stem and roots, etc.
The food manufactured by the leaves of a plant is transported to its all other parts through a kind of tubes called phloem (which are present in all the parts of a plant). The transport of food from the leaves to other parts of the plant is called translocation. Thus, phloem translocates the food (or sugar) made in the leaves. The movement of food materials (and other substances like hormones) through phloem depends on the action of living cells called sieve tubes.
Like xylem vessels, phloem is made of many cells joined end to end to form long tubes. However, the end walls of the cells which form phloem are not completely broken down. The end walls of cells in the phloem form sieve plates, which have small holes in them. These holes in the sieve plates allow the food to pass along the phloem tubes. The cells of phloem are called sieve tubes (or sieve elements).
Sieve tubes which form phloem are living cells which contain cytoplasm but no nucleus. The sieve tube cells do not have lignin in their walls. Each sieve tube cell has a companion cell next to it. The companion cell has a nucleus and many other organelles. Companion cells supply the sieve tubes with some of their requirements.
The food is made in the mesophyll cells (or photosynthetic cells) of a leaf. The food (like sugar) made by the mesophyll cells of a leaf enters into the sieve tubes of the phloem. Interconnected phloem tubes are present in all the parts of the plant. So, once the food (like sugar) enters the phloem tubes in the leaves, it is transported (or carried) to all other parts of the plant by the network of phloem tubes present m all the parts of the plant like stem and roots.
The translocation (transport of food from leaves to other parts of the plant) is necessary because every part of the plant needs food for obtaining energy, for building its parts and maintaining its life. Please note that when food is transported in a plant through a network of phloem tubes, then other substances made by the plant (like hormones) are also carried by the phloem tubes from one part of the plant to its other parts.
We have already studied that the movement of water (and dissolved salts) in xylem is always upwards (from soil to leaves) and it is caused by the suction of water at the top because of low pressure created by transpiration from leaves. The movement of food in phloem can be, however, upwards or downwards depending on the needs of the plant. We will now describe how food moves in the phloem tissue of a plant.