Plant Tissues: Meristems, Protective, and Supportive Tissues
Primary Meristem
Primary meristem is located at the tips of young plant organs: the apex of the root, stem, buds, etc., where it is the cone of plant growth. It provokes these organs to grow in length.
Root
Distinct parts are: suberized zone, absorbent hairs zone, and root cap zone. The growth zone is the subterminal region where the root meristematic tissue is located, causing an increase in the organ’s length. You can find 1, 2, or 3 meristematic cells. If there is 1 cell, this cell originates all the root. If there are 3 cells, they multiply to originate: the lower cells form the cap and epidermis, the middle cells form the cortex, and the superior cells form the central cylinder.
Stem
The stem grows in the opposite direction to the root. It is formed by knots, internodes, and buds.
Buds
Buds are small shoots where meristematic cells originate branches from the stem, or form leaves or flowers. Buds can be terminal or lateral.
- Terminal buds: Located at the end of the stem.
- Lateral buds: Located at the axillary angle where the leaf is inserted into the stem.
The meristem of the terminal bud causes the stem to grow in length. The higher cells cause the epidermis, the middle cells originate the cortex, and the lower cells form the central cylinder.
Secondary Meristem
Secondary meristem cells are located in older organs and appear after the first year of life. The cambial meristem determines growth in thickness. The accidental meristem forms when stems are cut or injured, to heal them.
Protective Tissue
Protective tissue covers the entire plant. It can be found on the upper and/or lower side of the leaf. It protects, mitigates, or eliminates the harmful effects of atmospheric variations.
Epidermal Formations
- Hairs: Elongated and branched cells. Found in stems and leaves, they secrete products for protection. Example: Urticating hairs of nettles secrete a caustic liquid.
- Stomata: Microscopic holes scattered in the epidermis with chlorophyll. They are kidney-shaped cells linked to form the ostiole. They can be found on the upper and/or lower side of the leaf. They act in gas exchange.
Suberous Tissue
Suberous tissue is formed by layers of dead or living cells. These cells are suberized, forming suber, making them impervious to liquids and gases. The cells are square and flattened. They are found in stems. Their function is to protect.
Supportive Tissue
Collenchyma and Sclerenchyma
| Collenchyma | Sclerenchyma |
|---|---|
| Living cells | Dead cells |
| Thin, prismatic cells | Short cells (sclereids) |
| Flexible cell membrane, heavy in the corners | Lignified and hard membrane, uniformly thickened, provided with pores |
| Found in young stems, branches, petioles, flower stalks | Found in permanent organs: old stems or branches |
| (Non-lignified fibers) | (Lignified fibers) |
Collenchyma tissue function: To provide flexibility and resilience. It is found in the petiole of leaves, flower stalks, young stems, and branches.
Sclerenchyma tissue function: To provide support. It constitutes the hardest parts of the plant. It is found in the shell of almonds and coconuts, in beans, etc.
Fibers
Fibers are elongated and spindle-shaped cells. Their cross-section is prismatic. The membrane is thick and can be lignified or not.
- Wood fibers: Short and rigid, their membranes are embedded in lignin. They form clusters around the wood vessels, which constitute the timber or firewood, the central and sturdy part of stems.
- Non-lignified fibers: They are longer than lignified fibers. They remain cellulosic, like the membranes, are flexible, and have great industrial application.