Plant Cells and Tissues

Plant cells are the basic units of life in plants, eukaryotic and photosynthetic organisms in the Plantae kingdom.
They are characterized by a cell wall made of cellulose, hemicellulose, and pectin, the presence of chloroplasts for photosynthesis and starch storage, a large vacuole, and unique cell division using a cell plate.
Plant cells can measure between 10 and 200 µm and are very different from other eukaryotic cells.

Plant tissue is made up of specialized cells derived from meristematic cells.
Cells within a plant tissue are classified based on their cell wall structure and protoplast characteristics.
These tissues form the roots, stems, leaves, flowers, and reproductive structures.

The epidermis, a specialized tissue made of parenchyma cells, covers the external surfaces of leaves, stems, and roots.
It contains guard cells for gas exchange, trichomes, and root hairs.
The epidermis of stems arises from the outer layers (L1 and L2) of the shoot apex, while the cortex and vascular tissue arise from the inner layer (L3).
Root epidermis originates from the layer of cells directly below the root cap.
All aerial organs, except roots, have an epidermis covered by a cuticle made of cutin or a cutan polymer, with a surface layer of epicuticular waxes.

Vascular tissue is responsible for transporting substances throughout the plant.
Xylem transports water and minerals.
It consists of tracheids or vessel elements, fibers, and parenchyma cells.
Tracheids are elongated cells with lignified secondary walls in their cell walls and are specialized for water conduction.
Tracheids appeared in plants during the Silurian period, over 425 million years ago.
Plants lacking or containing tracheids are termed non-vascular or vascular plants.
Tracheids are pointed and elongated, with either continuous primary walls or thickened secondary walls in the form of rings, spirals, or reticulate patterns.
More complex tracheids with valve-like perforations called bordered pits characterize gymnosperms.
Ferns and other pteridophytes, as well as gymnosperms, only have tracheids, while flowering plants also have xylem vessels.
Vessel elements are cells with perforated end walls to allow continuous water flow.

Phloem transports sugars, amino acids, and other organic compounds.
It consists of sieve tube elements, companion cells, fibers, and parenchyma cells.
Sieve tube elements are elongated cells with perforated end walls, lacking nuclei.
Companion cells support sieve tube elements metabolically.

Basic units of plant organisms
Found in green plants, eukaryotic, photosynthetic organisms of the Plantae kingdom
Typically contain a nucleus surrounded by cytoplasm, various organelles or plastids, all protected by a plasma membrane
Distinctive features include:
- Primary cell walls containing cellulose, hemicellulose, and pectin
- Presence of plastids capable of performing photosynthesis and storing starch
- A large vacuole that regulates turgor pressure
- Absence of flagella or centrioles, except in gametes
- A unique method of cell division involving the formation of a cell plate or phragmoplast that separates the new daughter cells
Can measure between 10 and 200 µm
Plant cells are living systems.
They are very different from the cells of organisms belonging to other eukaryotes.

Plant cells possess a nucleus containing most of the cell's DNA and mitochondria, the "factories" of the cell
Some parts of a living plant's tissues are selectively dead to serve the plant without consuming nutrients

Cells differentiate from undifferentiated meristematic cells (analogous to animal stem cells) to form the primary cell types and tissues of roots, stems, leaves, flowers, and reproductive structures, each of which can consist of several cell types, such as parenchyma, collenchyma, sclerenchyma, and vascular cells
All plants have these tissue types found in the same structures regardless of species
Classified based on their cell wall structure and protoplast characteristics

Specialized tissue composed of parenchyma cells, which covers the external surfaces of leaves, stems, and roots
Several types of cells can be present in the epidermis, including:
- Guard cells that control the rate of gas exchange between the plant and the atmosphere
- Glandular and hair-like trichomes
- Root hairs of primary roots
In the shoot epidermis of most plants, only guard cells possess chloroplasts
Chloroplasts contain the green pigment chlorophyll necessary for photosynthesis
Epidermal cells of aerial organs come from the superficial cell layer known as the tunica (L1 and L2 layers), which covers the plant shoot apex, whereas the cortex and vascular tissues come from the innermost layer of the shoot apex known as the corpus (L3 layer)
Root epidermis originates from the cell layer immediately beneath the root cap
The epidermis of all aerial organs, but not roots, is covered with a cuticle made of a polyester cutin or cutan polymer (or both), with a surface layer of epicuticular waxes
Shoot primary epidermal cells are believed to be the only plant cells with the biochemical capacity to synthesize cutin.