The pericycle is a unique layer of cells found within plant roots, and in certain instances, within their stems. This tissue plays a significant role in plant development, serving as a site where new structures can originate. Its activity contributes to plant growth and adaptation. Its functions are fundamental to the architecture and persistence of vascular plants.
Location and Cellular Structure
The pericycle is the outermost layer of the vascular cylinder, also known as the stele, within the plant root. It lies internal to the endodermis, which regulates substance movement into the stele. This places the pericycle near the xylem and phloem tissues.
The pericycle is composed of parenchyma cells, which are undifferentiated and can divide and specialize. These cells form a continuous, cylindrical sheath encircling the central vascular bundles. In some species, the pericycle also contains sclerenchyma cells, which provide structural support.
Formation of Lateral Roots
The pericycle is the origin point for new lateral roots, a process that begins internally within the parent root. Hormonal signals, particularly auxin, accumulate in specific pericycle cells, often opposite the xylem poles. This elevated auxin concentration triggers these cells to de-differentiate, meaning they revert to a more embryonic state, and then resume active cell division.
These initial divisions form a lateral root primordium. The primordium grows through repeated mitotic divisions, developing into a new root tip with its own root cap. As the nascent lateral root expands, it breaks down cells in the overlying tissues of the parent root, including the endodermis, cortex, and epidermis. This internal initiation ensures the newly formed lateral root maintains a vascular connection with the parent root’s stele as it emerges into the soil.
Contribution to Secondary Growth
Beyond initiating new roots, the pericycle also contributes to the secondary growth of roots, which is the increase in their girth or thickness. This function is prominent in dicotyledonous plants and gymnosperms, which undergo secondary thickening. Parts of the pericycle retain their meristematic potential, meaning they can form new meristems.
Segments of the pericycle, opposite the phloem bundles, become meristematic and contribute to the formation of the vascular cambium. This vascular cambium produces secondary xylem internally and secondary phloem externally, leading to the thickening of the root’s vascular core. Additionally, in many woody roots, the pericycle gives rise to the phellogen (cork cambium). The phellogen produces the periderm, a protective outer layer that replaces the epidermis as the root expands.
Variations in Different Plant Types
The roles and characteristics of the pericycle can vary between different plant groups, reflecting their distinct growth patterns. In most monocotyledonous plants, such as grasses and lilies, the pericycle’s primary function is limited to initiating lateral roots. Monocots do not exhibit extensive secondary growth in their roots, meaning their pericycle does not contribute significantly to the formation of vascular or cork cambia for thickening.
The presence and distinctness of a pericycle layer in plant stems are also more variable compared to roots. While a pericycle-like region may exist in some stems, its developmental significance is less pronounced than its role in root architecture and growth. These variations highlight the adaptive nature of plant tissues to different growth strategies and environmental demands.