Pericycle is a plant tissue found in the inner part of roots. It is located between the endodermis, which is the innermost part of the thick external tissues, and the phloem and xylem, which are the vascular tissues responsible for the transport of nutrients and water.The function of the pericycle is to form lateral roots, which increase water and mineral absorption in a plant.
Pericycle cells are very active, dividing quickly, especially during the growth season. The division of pericycle cells initiates the formation of new lateral roots, present in most plants. During this process, the pericycle cells push through several cell layers that form the root, including the endodermis. The structure that originates a new lateral root is called lateral root primordium.
The cells of the pericycle are narrower than those forming the endodermis. They also have thinner walls, which are structures protecting the cell membrane. Pericycle cells differ in size, according to the plant species, but also according to their specific location. In some species of plants, such as Queen Anne's lace, the pericyclic cells located near the xylem are shorter than those lying near phloem tissues.
Pericycle cells start to divide to form lateral roots because the plant produces growth hormones, such as auxines. This class of plant hormones is also responsible for the pericycle cell elongation, which occurs before the division. However, the interaction of auxins with other plant hormones, such as cytokinin and abscisic acid, can reduce the division rate in pericycle cells, thus decreasing lateral root formation.
Species Without Pericycle
Plant species lacking pericycle are very rare, and are often native of the Southern Hemisphere. Some members of the family Centrolepidaceae, found in Australia, New Zealand and South East Asia, do not have perycicle cells in their roots. These plants are grasses and develop all their roots from a common growing tissue.