Website Notes: Plant Tissues
I. Ground tissue - constitutes most of the primary body of a plant. It occurs throughout the plant and provides several important functions, including storage, basic metabolism, and support. These functions are performed by three constituent cells: parenchyma cells, collenchyma cells, and sclerenchyma cells.
A. Parenchyma cells (GR: para - "beside"; en + chien: "to pour in")
1. Basic functions:
b. Basic metabolism - parenchyma cells are the primary sites of metabolic functions including photosynthesis, respiration, and protein synthesis.
c. Ability to redeifferentiate or dedifferentiate. That is, they can change activities and become more specialized. E.g. wounding often stimulates parenchyma cells to divide and form masses on undifferentiated cells from which roots develop on a cutting. The wound induces parenchyma cells near the cut surface to dedifferentiate into meristematic cells, which then divide and produce root apical meristems that form roots on the cutting.
d. Parenchyma cells are made by a plant's meristems and occur throughout the body.
e. Specialized parenchyma cells
a. Chlorenchyma cells are chloroplast-containing parenchyma cells specialized for photosynthesis
b. Aerenchyma, another specialized parenchyma tissue, which is characterized by prominent intercellular space. These spaces improve the gas-exchange capacity of tissue for photosynthesis.
Take home point of parenchyma - these cells are the primary components of ground tissue and are the most abundant and least structurally specialized cells in plants. Parenchyma cells are the site of the basic functions of plants. These cells can form other, more specialized tissues.
B. Collenchyma (Gr: kolla - "glue")
1. Collenchyma cells are elongate cells with unevenly thickened primary cell walls; the walls are thickened in the corners of the cells.
2. They do not occur uniformly throughout the plant, but they support growing regions of shoots and are therefore common in expanding leaves and stems.
3. These cells are adapted for support, and provide flexible support in soft, nonwoody plant organs.
4. Collenchyma cells often differentiate into strands or as a cylinder beneath the epidermis. This maximizes support
5. Collenchyma cells differentiate from parenchyma cells.
a. Their differentiation is strongly influenced by mechanical
C. Sclerenchyma (Gr: skleros - "hard". Arterial sclerosis - hardening of the arteries).
1. Rigid cells that provide support and strength to non-extending regions of plants, such as mature stems. They occur in all mature parts of plants: leaves, stems, roots, and bark.
2. They are usually dead at maturity. This support is provided by the cell wall "skeleton", which is produced before the cell dies.
3. There are 2 types of sclerenchyma cells: sclereids and fibers, both of which differentiate from parenchyma.
Take home point - Collenchyma and sclerenchyma cells are thick-walled cells specialized for support. Collenchyma cells support growing regions, and sclerenchyma cells support nongrowing regions.
II. Dermal Tissue - covers the entire plant body and provides a covering over plant parts. In herbaceous plants (primary plant body), the plant covering is called epidermis.
A. Epidermis - the epidermis has several functions, including absorption of water and minerals, secretion of cuticle, protection against herbivory, and control of gas exchange. Comprised mostly of parenchyma cells with scattered guard cells.
1. Cuticle - the outer walls of epidermal cells are covered with a waterproof cuticle made of a fatty material called cutin.
2. Epidermal cells and gas exchange
a. The only intercellular spaces in the epidermis are stomata (GR: stoma - "mouth"). Stomata are small pores in the epidermis of leaves and stems, enabling gas exchange. Most abundant on the undersides of lvs.
b. Each stoma is surrounded by 2 guard cells (specialized cells of the epidermis), which are kidney-shaped (dicots) or dumbbell-shaped (monocots). They function by regulating the exchange of gases by opening and closing the stomatal pore. Opening and closing of stomata regulates the diffusion of CO2 into the leaf for photosynthesis. When CO2 enters a leaf thru stomata, water leaves. Indeed, for every gram of carbon fixed via photosynthesis, the plant loses between 250 - 600 g or water, depending on the type of photosynthesis. Losses of water thru stomata are an inevitable cost of photosynthesis. Paradox of the pores.
c. Stomata open and close in response to environmentally induced changes in turgor pressure of guard cells. Essentially, when a plant is well-hydrated, water is drawn into the guard cells. The guard cells elongate in response to this surge of water and, as they elongate, they bow apart and open the stomatal pore. Similarly, stomata close when water leaves the guard cells, which collapse upon themselves shut.
d. Point is that stomata are a critical adaptation for conserving water and therefore for maintaining life on land.
3. Trichomes - single or multi-celled outgrowths of epidermal cells (hairs or scales).
a. Root hairs (simple, unbranched trichomes) are outgrowths of epidermal cells specialized for absorbing water and minerals from the soil. They occur near the tips of roots where they may be as abundant as 40,000/cm2. The root hairs increase the absorptive surface area of roots several thousand-fold, thereby enabling the plant to extract water and dissolved minerals more effectively from nooks and crannies in the soil.
b. Protection - in many plants, trichomes deter animals and insects and, is some plants, humans.
The epidermis is relatively short-lived in many plants. Radial expansion of the stems usually ruptures the epidermis during the first year of growth. When that occurs the epidermis is replaced by a secondary dermal tissue, called periderm (the protective outer bark of older roots and stems).
Take home point - The epidermis covers the primary body of a plant. It is covered with a waxy cuticle that decreases water loss and is perforated by pores, called stomata, that control gas exchange. Outgrowths of epidermal cells called trichomes have many functions, including absorption and protection.
III.Vascular tissues: Xylem and Phloem
Vascular tissues are specialized for long-distance transport of water and dissolved solutes. They ramify throughout the plant, but are particularly evident in the leaves; they are the veins you see in a leaf.
Xylem and phloem are the 2 kinds of vascular tissues in plants.
A. Xylem - (Gr: xylos - "wood") - transports water and dissolved nutrients in an unbroken stream from the roots to all parts of the plant. The water transported in xylem replaces the water lost via transpiration through stomata. Xylem also provides structural support for plants.
1. The cells responsible for water conductance (movement) in xylem are called tracheids and vessel elements.
2. Tracheids, the chief water conducting cells in gymnosperms (conifers) and seedless vascular plants (ferns), are long, overlapping, tapering cells. Water is conducted from one tracheid to another thru bordered pits, thin areas with in the cell walls.
3. Vessel elements are found in flowering plants (angiosperms), are more advanced evolutionarily, and are very efficient water conducting cells. Vessel elements have larger cell diameters than tracheids, and they are stacked one on top of the other, with perforations in their end walls. They resemble miniature water pipes. (They are both shorter and wider than tracheids). They move a greater volume of water thru their cells, at a faster rate than do tracheids.
Take home point - Xylem conducts water and dissolved minerals from roots to leaves. Water moves through xyllary elements called tracheids and vessel elements, which are dead, hollow cells having thick secondary walls.
B. Phloem (Gr: phloios - "bark") - conducts food materials - carbohydrates formed in photosynthesis, throughout the plant, and provides structural support.
1. Food materials are conducted in solution - dissolved in water.