Primary Secondary Growth Region of Growth Points In Plot Stem and Root

Plant Growth 

Plant growth can be differentiated into two: primary and secondary growth. Primary growth is growth as a result of cell division in primary meristematic tissues. On the other hand, secondary growth is the result of activities of the secondary meristematic tissues, i.e. cambium and cork cambium. 

Primary Growth 

Primary growth in plants only occurs on several plant parts, such as actively dividing and growing parts. These parts are meristematic tissues. There are two types of primary meristematic tissues: shoot apical meristem and root apical meristem. 

Root apical meristem 

Root apical meristem is the tip part of a root that has a root cap (calyptra). The root cap functions to penetrate the soil. On this growth region, active meristematic tissue is found. 

Based on the meristematic tissue structure of the composing cells, the growth region of plant roots can be differentiated into the regions of cell division, cell elongation, and differentiation. The dividing region is located at the root tip. At this region, cells divide instantly. At the elongation region, cells undergo elongation and start structural differentiation. They become protoderm, ground meristem, and procambium. Protoderm will eventually become the epidermis. Ground meristem will be formed into ground tissues. Procambium tissue will form stele (central-part cylinder). Look at .Figure 1.6. At the differentiation region, organogenesis has run to form a clearly-differentiated epidermal layer that has root hairs. Root hairs function to absorb minerals and water from the soil. Since the first differentiation process occurs on this region, differentiation region is called primary tissue. 

Growth will cause root cells to elongate. 

Shoot apical meristem 

Shoot apical meristem is meristematic tissue at the tip of the stem to grow major stem or tree trunk. It can be observed on plants that have a shoot tip in the form of a bud. Meristematic tissue divides to form a domeshaped mass. Primordia will grow from the side of this dome-shaped mass. Lateral buds will form branches. 

The stem also has a region of cell elongation Here, formed tissue also undergoes a differentiation process just as the root does. 

Stem meristematic tissue is of two types: embryonic meristem and cambium meristem. Embryonic meristem is found during germination. Cambium meristem is found in grown and perfectly developed plants. Look at Figure 1.7. 

Secondary Growth 

After undergoing primary growth, plants continue with secondary growth. Secondary growth usually tesults in enlargement of the stem diameter. However, only Gymnosperms (plants that have naked seeds) and dicotyledons experience secondary growth. Monocots lack this type of growth, except for palms (Palmae). 

Secondary growth can be observed at every stage of the plant’s growth and development. The stage where roots and stems elongate is the primary growth stage. The next stage where the stem gets larger is the secondary growth. 

During secondary growth, actively dividing cells are meristematic cells in cambium. Cambium is sandwiched between xylem and phloem. Cells divide in radial orientation, cells close to xylem divide in the inward direction while those next to phloem divide outwardly. This region is called cambium meristem. This growth forms secondary xylem and phloem (Figure 1.8). The growth of this tissue will form a circular structune called annual rings. 

Cambium activity that forms secondary xylem and phloem is often imbalanced compared to the growth of the plant’s bark. This condition results in ruptured epidermal tissue and outer cortex. This can harm the inner tissues. Plant overcome it by producing cork cambium (phellogen) or cork tissue that forms phelem in the outward direction and pheloderm in the inward. Phellem (cork layer) is a layer of dead cells, as opposed to living cells in phelloderm (secondary cortex). There are slits along cork tissue called lenticels. They function as passage for the entrance of water and air into plant cells. 

FIGURE 1.8 Cross section of a tree trunk showing secondary growth. The tree is more than three years old. 

Important Concept 

Primary growth is the lengthening of stem and roots as an activity of meristematic cells on growth point at stem and roots. Secondary growth is diametric growth of the stem as an activity of meristematic cells in cambium.

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