Plants exhibit an astonishing range of growth patterns, from the rapid upward reach of a sunflower to the gradual thickening of an ancient oak. While many familiar garden plants primarily grow taller, a distinct process allows some to expand in girth, developing the substantial structures that define trees and shrubs. This lateral expansion, distinct from vertical growth, enables plants to build robust frameworks over many years.
What is Secondary Growth?
Secondary growth in plants refers to the increase in thickness or girth of stems and roots, contrasting with primary growth, which causes elongation at the tips. This growth defines woody plants like trees, shrubs, and some vines, giving them their woody texture and strong trunks. Most herbaceous plants do not undergo significant secondary growth. Its primary function is to provide structural support, allowing plants to grow larger and taller by strengthening their architecture to withstand environmental forces like wind and gravity.
The Tissues Responsible
The increase in plant girth is orchestrated by specialized regions of continuously dividing cells called lateral meristems. Two primary lateral meristems are responsible for secondary growth: the vascular cambium and the cork cambium. These tissues ensure the plant can expand radially while maintaining its internal transport and external protection systems.
The vascular cambium forms a cylindrical layer, typically located between the existing xylem and phloem tissues within the stem and root. Its cells divide to produce new tissues both inward and outward. Inward divisions generate secondary xylem, commonly known as wood, which accumulates over time. Outward divisions produce secondary phloem, contributing to the inner layers of the bark.
Secondary xylem is important for the efficient transport of water and dissolved minerals from the roots throughout the plant, and provides structural support. Secondary phloem transports sugars produced during photosynthesis from the leaves to other parts of the plant, including roots and growing tissues.
The cork cambium, also known as phellogen, forms in the outer regions of the stem or root. This meristem produces cork cells, or phellem, towards the outside and phelloderm cells towards the inside. The cork cells are dead at maturity and become impregnated with suberin, a waxy substance that makes them waterproof and protective.
Collectively, the cork cambium, phellem (cork), and phelloderm form the periderm, which replaces the epidermis as the plant’s outer protective layer. As the plant’s girth increases, the original epidermis often ruptures, necessitating the formation of this new protective tissue.
Visible Manifestations and Importance
The most prominent manifestation of secondary growth is the formation of annual growth rings, visible in a cross-section of a tree trunk. These rings represent a single year’s growth and arise from the seasonal activity of the vascular cambium. During periods of rapid growth, typically spring, the vascular cambium produces larger, thin-walled xylem cells (earlywood or springwood), which appear lighter. In contrast, slower growth periods, such as late summer or autumn, result in smaller, thicker-walled xylem cells (latewood or autumnwood), appearing darker. The width of these rings can provide insights into past environmental conditions, with wider rings often indicating favorable growing seasons.
Bark Formation
Another visible outcome is the development of bark, the outermost protective layer of woody stems and roots. Bark comprises accumulated layers of secondary phloem and the periderm. As new wood forms, older outer tissues are pushed outward, often cracking and flaking, forming distinctive patterns on tree trunks.
Importance of Secondary Growth
The significance of secondary growth extends to its important roles in plant survival and longevity. It provides structural support for tall plants, allowing them to grow upright and maintain their form against physical stresses. The vast network of secondary xylem facilitates the long-distance transport of water and nutrients, supporting the metabolic demands of a large plant body. Protective bark shields the plant from desiccation, pests, diseases, and temperature fluctuations.