Secondary Growth

Secondary Growth

Plants, like other living organisms need to grow, and they do this by mitosis. Cell growth is limited to the specialised cells that make up meristematic tissue. Meristematic tissue consists of small cells that are unspecialised. These cells divide by mitosis to form new cells that can differentiate i.e. undergo changes in their structure. When groups of cells differentiate they form specialised tissue (e.g. xylem, phloem, epidermal cells). There are different types of meristematic tissue:

• Primary meristematic tissue is found in the tips of roots, stems and buds. When the cells divide, new cells are produced which causes the plant to grow in length. This is referred to as primary growth.
• Secondary meristematic tissue originates from permanent tissue, usually parenchyma tissue which divides by mitosis. Cambium is secondary meristematic tissue that is found in roots and stems. When these cells divide by mitosis it results in the plant becoming wider. This is called secondary growth.

Secondary growth is seen clearly if you examine the stump of a tree. During every growing season the stem of a plant increases in width. This is known as secondary thickening. Towards the end of the first year of growth, the parenchyma cells between the vascular bundles become meristematic. This means that they actively start dividing by mitosis, and link up with the cambium that occurs in the vascular bundles to form a cambium ring in the stem (see the figure below). The cells in the cambium ring start dividing to form secondary phloem (on the outside of the cambium ring) and secondary xylem (on the inside of the cambium ring). Each year another ring of secondary phloem and secondary xylem is formed, making the stem grow wider.

Process of secondary thickening in stems.

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It is not possible to see the layers of secondary phloem but the secondary xylem layers are visible. These form rings called annual rings which can be used to work out the age of a plant.

As new rings are formed each year, the older rings are pushed inward and the xylem vessels collapse due to the pressure. The wood in the centre becomes denser and harder than the wood at the surface and is called heartwood. The youngest annual rings found on the outside transport water. This wood is not as dense and is called sapwood. The light-coloured rings are called spring wood. They are formed during spring and summer when the growing conditions are favourable. These rings are therefore relatively broad and light in colour as the xylem cell walls are thin. The dark-coloured rings are called autumn wood. They are formed during autumn and winter when the growing conditions are unfavourable. The rings are therefore relatively narrow and dark in colour as the xylem cell walls are thick. Rings may be thicker in some years than in others, depending on the amount of rain and nutrients received.

There is another type of cambium in plants called cork cambium. Cork cambium forms when an outer layer of parenchyma in the cortex becomes meristematic and produces cortex cells to the inside and cork cells to the outside. This forms the dry outer cells of the bark on a stem.

The annual rings in a tree trunk give a measure of the tree's age as well as the environmental conditions of the tree's growth.