A plant grows when its stem and roots expand or, in other words, when the tissues in the plant expand or multiply, hence increasing the plant growth.Stems are a subsection of the shoot system; they are of various shapes, lengths, and girths. Stems are differentiated based on the growth of nodes and internodes.The stem is mainly responsible for transporting nutrients or by-products of photosynthesis throughout the plant.Like the roots, the plants also grow in 2 ways, primary and secondary growth.Primary growth is when the stem elongates; it stretches, resulting in plant elongation. It happens in the meristem.After primary growth is secondary growth. In secondary growth, the girth or width of the stem increases; it thickens. Secondary growth occurs in the secondary meristem, vascular cambium, or cork cambium. This layer gives the woody texture of the bark.Secondary growth may be little or almost nil in some plants. Secondary growth mainly occurs in dicotyledon plants and is extremely rare in monocotyledon plants.
Anatomy of Plants
The prerequisite anatomy for every stem must include these tissues: the epidermis, cortical vascular bundles, and medullary region.
Collenchyma tissues are responsible for structural support.
Parenchyma tissues perform photosynthesis, i.e. they are responsible for metabolism, storage, and repairing tissues.
Sclerenchyma tissues are also for structural support but are dead when mature.
The epidermis is a single cell that covers the whole stem, and it is a very thick layer that has cutin in the free cell wall. The cortex is a layer of parenchyma cells right under the epidermis; these cells either perform photosynthesis or store food. The cortex exterior is made up of either sclerenchyma or collenchyma.
The cortex also contains an endodermis which performs separately from the rest of the cortex; this region, in some cases or species, has a Casparian Ring.
Vascular bundles are of two types with different functions; they are known as xylem and phloem. Xylem is found in the exteriors, whereas phloem is found in the interiors.
The distribution of these tissues is random in monocot plants. In dicot stems, the tissues are arranged in a ring.
- Phloem comprises sieve-tube cells, companion cells, phloem parenchyma, and phloem fibres. It transports all organic substances.
- Xylem – tissue is composed of xylem parenchyma, tracheids, and vessel elements. The parenchyma and tracheids are responsible for water transport; the vessel elements help in the transportation of water upwards.
Primary growth in stem
Cell division in the apical meristem is the primary source of growth in the stem. Primary growth generally occurs at the tips of stems and roots, also known as apices. The rapid dividing of cells present in the apical meristem at the shoot and the root tip is called primary growth.As a consequence of the divisions in the apical meristem, cell elongation also occurs, which contributes to primary growth. Without primary growth causing growth in roots and shoots, plants can consequently seek more water and sunlight.
A great example of apical dominance can be observed in the Christmas tree, a coniferous tree. It is due to apical dominance that coniferous trees get their shape. If the apical buds were to be pruned, the plant would start forming lateral branches using the axillary buds. This fact is used in gardening to develop bushes of the same height and thickness.
Secondary growth in stem
We know that plants have many meristematic tissues in places with higher primary growth, namely the stem and the root. These tissues are called the apical meristematic tissues, but there is another set of meristematic tissues on the sides of the plant, known as the lateral meristematic tissues.
The role of the lateral meristematic tissues is to increase the diametrical size of the stem. In many woody plants, the lateral meristematic tissue constitutes most of the plant. The primary responsibility of lateral meristematic tissues is to provide support for the plant during its growth and protect it from the effects of the elements. Lateral meristem tissues also partake in the conduction of water and nutrients.
The growth of this form is called secondary growth in plants. Lateral meristems, vascular cambium, and cork cambium are different tissues that partake in the secondary growth of the plant.
The vascular cambium produces secondary vascular tissues, whereas the cork cambium is responsible for periderm. Seeds that are gymnosperms and/or dicots usually exhibit secondary growth like trees and shrubs.
Conclusion
Stems in plants grow in 2 ways, namely primary and secondary growth.Primary growth is when the stem elongates; it stretches, resulting in plant elongation, and it happens in the meristem.Cell division in the apical meristem is the primary source of growth in the stem. Primary growth generally occurs at the tips of stems and roots, also known as apices. The rapid division of cells present in the apical meristem at the shoot and the root tip is called primary growth. The set of meristematic tissues on the sides of the plant is known as the lateral meristematic tissue. The role of the lateral meristematic tissues is to increase the diametrical size of the stem. In many woody plants, the lateral meristematic tissue constitutes most of the plant. The primary responsibility of lateral meristematic tissues is to provide support for the plant during its growth and protect it from the effects of the elements. Lateral meristem tissues also partake in the conduction of water and nutrients. The growth of this form is called secondary growth in plants.