Lab 2 Flashcards
Transpiration
Transpiration
Transpiration is the loss of water due to evaporation from leaves and other aerial portions of the plant. Most water loss occurs when water evaporates from the mesophyll cells within the leaf and exits the leaf via the stomata (singular stoma). Transpiration is also responsible for the upward movement of water through the xylem by exerting a pull from above on the continuous column of water within the xylem tubes extending from the roots to the leaves.
Stomata
Plants can regulate the rate of transpiration by opening and closing the stomata. Stomata are normally open during photosynthesis to allow carbon dioxide to diffuse into the internal air spaces of the leaf and are closed when photosynthesis is not occurring (i.e. at night to prevent excessive water loss from leaves). In the conifer plant the stomata are found on the underside of the leaves.
Transpirational Pull
Loss of water due to evaporation from the leaves and other aerial parts of the plant. Causes an upward pull of water (continuous column) within the xylem tubes extending from the roots to the leaves.
Potometer
Measure transpiration indirectly (measures volume of water taken up by branch). The principle is that every molecule of water lost by transpiration is replaced by equal amount of water being pulled by the xylem. The rate of xylem flow (bubble*) equals the rate of transpiration.
Roots
Roots are multicellular structures containing vascular tissue. A root system can grow quite large to absorb the amount of water needed for a plant. Roots also provide anchorage for the plant.
Epidermis
outer protective layer of cells.
Cortex
Beneath the epidermis, a relatively wide layer of parenchyma cells (large, thin walled). Most of these cells contain stored starch grains (stained pink).
Endodermis
Innermost layer of the cortex. It is a layer one cell thick with many of the cells showing thick walls. The endodermis forms a boundary between the cortex and the central vascular cylinder or stele and regulates movement of water and minerals into the vascular cylinder.
Vascular cylinder/ stele
composed of the pericycle, xylem and phloem.
Pericycle
Internal to the endodermis, a layer of thin-walled cells larger than the cells of the endodermis. This is a meristematic tissue which provides dividing cells that allow the root to grow in diameter and produce branch (lateral) roots.
Xylem
In the centre of the vascular cylinder containing many large hollow cells with thick, heavy (stained red) walls. The xylem tissue may be arranged in three to five radiating arms. Water and minerals absorbed by the root hairs are collected in the xylem for transport to the rest of the plant.
Phloem
Lies between the xylem arms and is composed of many small cells with cytoplasm and thin walls (stained green). Sugar and other organic nutrients are transported throughout the plant in the phloem.
Root hairs
Outgrowths of single epidermal cells and as such are non vascular. Root hairs provide an extensive surface for water and mineral absorption. Water moves through the cortex to the xylem for transport to the rest of the plant.
Herbaceous stem
An herbaceous stem is a non-woody stem or one which lacks secondary xylem and phloem.
Epidermal cells
Outermost layer of cells which provide protection for the underlying cells.
Vascular bundles
Arranged in a ring around the edge of the stem. Each bundle consists of xylem cells (transporting minerals and water), phloem cells (carrying photosynthetic products) and sclerenchyma cells (providing support). The part of the bundle located closest to the centre of the stem is made up of xylem cells. Most xylem cells are tracheids and vessels, both of which are thick walled and dead at maturity. External to the xylem is the phloem. Phloem cells are smaller than xylem and have relatively thin cell walls. Vascular bundles of Ranunculus also contain sclerenchyma cells. These cells form a ring around each bundle.
Ground tissue
The majority of the stem is composed of ground tissue which contains several types of cells. The primary cell type in ground tissue is parenchyma, an unspecialized cell type that carried out metabolic processes, stores organic products as well as develops into other specialized cell types.
Tracheids
Tracheids are long cells with tapering ends. They have a hollow cavity and thickened cell walls that contain pits (thin areas in the cell walls with cytoplasmic connections to adjacent cells).
Vessels
Vessels are chains of slightly elongated cells attached end to end. The end walls of which are partially dissolved so that vessels form long continuous tubes. These allow for the rapid transport of water up through the plant. Vessels are typically wider and shorter than tracheids. In addition to pits the cell walls are often strengthened by thickenings. The thickenings may appear as rings or irregular bars.
Central pith
Composed of large thin-walled cells that make up the inner most section of the stem.
Woody stem
A woody stem is one which possesses secondary xylem and secondary phloem.
