The internal structure of the root, stem and leaf system of plants is fascinating and allows us to have an understanding of how plants work. Though there are many variations the basic structure is the same.
Using a cross section, the internal structure of a root is like this:
FIrst there is the epidermis. This is a layer of protective square shaped cells. There are many projections in the epidermal layer. These are the root hairs. The epidermis of root hairs is thin and covered in mucus which makes them stick to soil particles and absorb water.
Going in the root from the epidermis you come to the cortex – packing cells which give shape and structure. In the centre is the Casparian strip – a narrow band of material chemically different from the rest of the tissue and it allows water in but not out.
The vascular tissue consisting of xylem and phloem tubes are grouped in the root’s centre.
Plants’ stems provide support and hold leaves up to sunlight. They also protect the plant’s water and food carrying systems- the xylem and phloem. Xylem carries water and minerals from the roots up the plant to the leaves and phloem carries food from the leaves to the rest of the plant. Xylem also provides support for plants and this is especially important in large plants such as trees.
Going across a stem section you first find the epidermis which acts as a water proof barrier and allows some water and gases in and out through stomata or lenticels. Then there is the cortex which is a layer of tissue just inside the epidermis. The cortex is mostly large, loosely packed cells with air spaces. It allows gases to circulate. The inner part of the stem has closer packed cells and is the pith.
The vascular bundles of xylem and phloem are found between the cortex and pith. Each bundle has of xylem on the inside and phloem on the outside with a layer of cambium between the two if the plant is a dicotyledon. The veins of a leaf are extensions of the vascular bundles of the stem.
In a cross section of a simple leaf, the internal structure is as follows.
The upper epidermis is a single layer of cells, usually transparent. Sunlight passes through these cells to the chloroplasts in the pallisade cells below. The epidermis keeps the leaf’s shape and protects the cells inside. Sometimes there is a waxy cuticle over the epidermis to help water retention and provide waterproofing.
Under the epidermis is the palisade layer, consisting of column shaped cells containing large numbers of chloroplasts. These contain chlorophyll, which absorbs energy from sunlight to help the plant make food.
Under the palisade layer is a layer of loose-fitting cells and air spaces called the spongy layer. This allows water vapour and gases to circulate freely. The palisade layer and spongy layer together form the mesophyll.
The lower epidermis usually contains tiny pores or holes known as stomata which open to allow carbon dioxide, water vapour and oxygen in and out of the leaf. Each stomata is edged by two crescent shaped guard cells which control the size of the stomatal pore..
