Transport in plants

Why plants need transport

A plant’s parts do different jobs in different places. The leaves make food but need water, which is in the soil. The roots take in water but cannot make food, because they are underground and have no chlorophyll. So water must travel up and food must travel to every part. Plants do this through two sets of thin, tube-like tissues: the xylem and the phloem.

Water and minerals: the xylem

Roots take in water together with minerals dissolved in it. Minerals are nutrients that plants need for healthy growth. The water and minerals then move up the stem, into the branches and leaves, through the xylem. Leaves use the water in photosynthesis and for many other jobs.

The red ink experiment

You can watch water rising. Take two tumblers, A and B, a third full of water, and add a few drops of red ink to B. Cut the stems of two similar tender twigs with white flowers, such as white sadabahar or balsam, at a slant while holding them under water, and place one in each tumbler. By the next day, the veins of the leaves and the petals of twig B show red streaks, while twig A looks unchanged.

Now cut across the stem of twig B above the water level and look at the cut end with a magnifying glass. Small red spots are visible: these are the xylem tubes through which the coloured water climbed. In the same way, minerals dissolved in water travel up the plant. (Extra: cutting under water keeps air bubbles out of the tubes, and much of the upward pull comes from water evaporating from the leaves through the stomata.)

Food: the phloem

Leaves are the main site of photosynthesis, but every part of the plant needs food. The food made in the leaves is carried to all parts through another set of thin tubes, the phloem. Some of it is used at once for energy and growth, and some is stored, for example in seeds and roots. The starch in a potato, the sugar in sugarcane and the food in grains all arrive through the phloem.

Two systems working together

The xylem and the phloem run side by side through the roots, stem and leaves, like two sets of pipes. Water and minerals go up in the xylem to the leaves; food made there goes out in the phloem to the rest of the plant. If either system is damaged, for example when a ring of bark is stripped from a tree trunk, the parts beyond the damage start to suffer. (Extra: the phloem lies just under the bark, so a stripped ring stops food from reaching the roots.)

Why it matters

Farmers water crops and add fertilisers to the soil because the roots and xylem must supply water and minerals to the leaves. The phloem then fills grains, fruits and tubers with food. See how the food is made in photosynthesis and how it is used in respiration in plants. Answers to the textbook questions are in the NCERT solutions for Chapter 10; see all of Chapter 10 notes.