Transport in plants
The leaf has a midrib which contains xylem and phloem vessels for the transport of water to the leaves and food from the leaves respectively.
- carries food from the leaves to all parts of the plant
- carries water and dissolved salts from the roots to the leaves.
- supports the plant
- spaces out the leaves so that they can efficiently receive sunlight and absorb carbon dioxide, needed for photosynthesis.
- elevates the flowers from the ground, to help pollination by insects or the wind.
The epidermis is a single layer of cells that helps to reduce the loss of water through evaporation. Woody stems have a bark made up of many layers of dead cells instead of the epidermis.
Vascular bundles are groups of specialised cells that transport water, dissolved salts and food along the stem. The vascular bundles consists mainly of xylem and phloem vessels. Phloem transports food and xylem transports water and salts.
Cortex and pith
The cortex is the tissue between the vascular bundles and the epidermis. The cells in the cortex store starch. The cortex of green stems contain chloroplasts and is able to make food by photosynthesis.
The pith is the central tissue of the stem. The pith and cortex together helps to support the stem.
In the root, the vascular bundle is in the centre, unlike the stem where the vascular bundles form a cylinder in the cortex. The xylem carries water and salts from the root to the stem, whilst the phloem carries food from the stem to the root, to provide the root with substances for energy and growth.
The root has no epidermis, and at the root tip there are several layers of continually growing cells forming the root cap. Just above the root tip, the outer cells produce tiny outgrowths called root hairs. The root hairs take up water from the soil by osmosis
and absorb mineral salts by active uptake. The large number of tiny root hairs increases the absorbing surface of a root system.
Water enters the root hair cells by osmosis into the cells in the root cortex. From there it enters the xylem vessels and then travel up the stem into the leaves.
Transpiration is the loss of water by a plant through the leaves. Transpiration creates a concentration gradient that draws water from the soil and through the plant as water evaporates from the leaves. This concentration gradient creates a flow of water called the transpiration stream.
The importance of transpiration
Most of the water taken up by the plant is lost through the leaves by transpiration. Transpiration is important to the plant because it:
- helps to cool the plant when it is exposed to intense sunlight.
- creates a transpiration stream that draws water and minerals up the stem.
Factors affecting the rate of transpiration
Transpiration is the basically the evaporation of water from the leaves which means that so any factor affecting evaporation will also affect transpiration the same way.
- Light intensity – In bright light, the stomata opens and allow the water vapour in the leaves to diffuse out into the atmosphere thereby increasing the rate of transpiration.
- Humidity – High humidity in the air reduces the rate of transpiration since the concentration gradient between the leaf and the air will be low.
- Wind – wind sweeps away water vapour from the leaf as fast as it diffuses out thereby speeding up the rate of transpiration.
- Temperature – Warm air around the leaf increases the rate of evaporation of water from the leaf and also the rate of transpiration.
To measure water loss by a plant
To measure the water loss by a plant we can use an instrument called a potometer.
A potometer is set up as shown in the diagram above. Transpiration from the leafy shoot causes water to be taken into the shoot from the apparatus and the air bubble moves to the left. The faster the rate of transpiration, the faster the air bubble moves. To reset the apparatus, the tap on the reservoir is opened to replace the water lost.