Plant Tissues, Organs, and Organ Systems

Just like animals, plants have organs which carry out specific functions. The leaf is an organ, made up of multiple tissues which play unique roles in photosynthesis.

 
 

Plant tissues

Tissues are groups of cells which work together to carry out a specific function. A collection of tissues which work together to carry out a function is called an organ. Plants possess the following tissues:

  • Epidermal tissues – the epidermis is the upper tissue layer of a leaf. It is thin and transparent which allows light to reach the photosynthesising cells in the palisade mesophyll layer below. Together with the waxy cuticle, it also protects the leaf from damage.

  • Palisade mesophyll – this is a tissue layer found towards the top of leaves where most of the photosynthesis takes place. Cells in the palisade mesophyll layer are found close together and are packed full of chloroplasts to absorb the light energy needed for photosynthesis. Their position towards the top of the leaf also means that they can absorb as much light energy as possible.

  • Spongy mesophyll – this is a tissue layer in the leaves which contains air spaces to allow gas exchange to take place. The carbon dioxide needed for photosynthesis can circulate and diffuse into cells while the oxygen produced from photosynthesis can diffuse out of the cells and into the air spaces and will eventually diffuse out of the leaf through the stomata.

 
 

  • Xylem and phloem – these are vessels which transport water and sugars around the plant. Xylem carries water (and dissolved mineral ions) from the roots to the rest of the plant. Phloem vessels transport sugars from photosynthetic regions of the plant (such as the leaves) to other parts of the plant.

  • Meristem tissue – this is found at the growing tips of roots and shoots and contains stem cells. Stem cells are unspecialised cells which can divide into any type of cell to form the various parts of a plant (for example, some stem cells will become root cells, others will become pollen grains). The meristems also contain cells which are actively dividing which allows the plant to grow.

Stomata

The leaf also contains stomata — gaps in the leaf which control gas exchange. They allow carbon dioxide to enter and oxygen to exit the leaf.

Guard cells are found at either side of stomata. At night, they lose water by osmosis and become flaccid. This closes the stomata and prevents transpiration when the plant is not photosynthesising. In the day, the guard cells gain water by osmosis and become turgid. This opens the stomata, allowing gas exchange to take place.

Stomata are mostly found on the bottom of the leaf. This reduces transpiration as the underside is shaded and cooler.

 
 

Plant organisation

Plants have two transport systems – the xylem and the phloem – which are adapted for transporting water and sugars. You need to know how these two transport vessels, along with root hair cells, are specialised for carrying out their function.

How it is adapted for its function

Root hair cells have a large surface area to volume ratio, which increase their uptake of water by osmosis. Their cell membranes have lots of carrier proteins for the active transport of mineral ions.

Xylem vessels transport water and mineral ions from the roots to the stem and leaves. They are composed of hollow tubes strengthened by lignin. Water moves through the plant due to transpiration.

Phloem vessels transport dissolved sugars from the leaves to the rest of the plant for immediate use or storage. They are composed of elongated cells which contain pores in their end walls to allow cell sap to move from one phloem cell to the next. The movement of sugars around the plant is known as translocation.

Plant cell / tissue

Root hair cell

Xylem tissue

Phloem tissue

Transpiration

Water moves into root hair cells by osmosis and is transported to different parts of the plants in the xylem. Water near the surface of the leaf evaporates and exits the leaf as water vapour through the stomata. This causes water to move up the xylem to replace the water lost from the leaves. This is known as a ‘transpiration stream’ or ‘transpiration pull’.

Factors which affect the rate of transpiration:

  • Humidity: at high humidity, the air is full of water vapour. This reduces the concentration gradient of water vapour between the inside and the outside of the leaf. In turn, this reduces the rate of transpiration as water vapour diffuses out of stomata more slowly.

  • Wind speed: the faster the wind speed, the faster the rate of transpiration since windy conditions will move any water molecules hanging around outside of the leaf. This increases the concentration gradient so increases the rate of transpiration.

  • Temperature: higher temperature increases the rate of transpiration as the water molecules have more kinetic energy so move faster out of the stomata. Higher temperatures also increase evaporation of water from a liquid to gaseous state.

  • Light intensity: higher light intensity causes the stomata to be open for longer time periods, therefore the amount of transpiration will increase.