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Describe the general leaf internal anatomy.
- Upper epidermis:
- Mesophyll: tissue that makes up the most of a leaf, and is made up of ground tissue specialized for photosynthesis. Found between the upper and lower epidermis it has large intercellular spaces which communicate with the stomata and numerous chloroplasts. The mesophyll tissue has two layers…
- Palisade parenchyma – tightly packed elongated cells usually perform the majority of photosynthesis (usually located at the top of the mesophyll layer)
- Spongy parenchyma – loosely packed cells that perform SOME photosynthesis (usually located beneath the palisade parenchyma)
- Lower epidermis:
- Note: typically no stomata are found in the upper epidermis
Describe the vascular tissue of a leaf
- The vascular bundles of the leaf are often called veins. Bundle sheaths often wrap around the outside of a vein. Two types of venation…
- Netted: (common to dicots) veins have tributaries (side veins) that branch from a larger midvein.
- Parallel: (common to monocots) veins run parallel to each other like a blade of grass
- Note – midvein + associated ground tissue = midrib. A midvein is JUST the vascular tissue.
Why do leaf structural adaptations occur? Describe the general types in detail.
- Leaf adaptation exists due to different environment conditions, with water availability being the most important factor.
- Hydrophytes: (water environment) leaves beneath the water have no stomata, leaves resting on water contain stomata only on the upper epidermis (can’t breathe water). Have reduced vascular tissue (especially xylem) because it resides in H2O. Epidermis is covered with a cuticle to keep water from escaping. Large intercellular space allows for buoyancy. Large sclereids keep the plant semi-rigid even with the large amount of intercellular space.
- Xerophytes: (arid environment) contain stomatal crypts (invaginations of lower epidermis) for trichomes and stomata. Upper epidermis is thick and multilayered to keep water in the plant. Have greater number of stomata than other leaves so that photosynthesis can occur rapidly when water conditions are favorable. Trichomes can be on either/both surfaces of the leaf (retards water loss). Palisade parenchyma often occurs on both sides of the leaf (maximizing photosynthetic potential).
- Mesophytes: (normal environment) as described in “general internal leaf anatomy.” Palisade parenchyma packed tightly near top of mesophyll with spongy parenchyma found below. Stomata and trichomes on bottom of leaf.
- Note – actual leaves could be a blend of these three types.
What is secondary growth (moderate detail) and what types of plants does it occur? Give information about secondary growth.
- In most herbaceous dicots and the monocots growth in a given region of a plant ends after the maturation of the primary tissues. (primary growth)
- In others, including woody magnoliids, woody dicots, and the gymnosperms growth in wide begins in regions that are no longer elongating. (secondary growth)
- Note - Primary and secondary growth occur simultaneously but in different regions of the plant.
- Secondary growth: lateral meristems (vascular cambium [from procambium] and cork cambium [from ground meristerm]) add girth by producing secondary vascular tissues and periderm.
- Secondary growth is found in roots and stems and occurs in all gymnosperms and most angiosperms (except monocots)
- Stem elongation is due to activity with the apical meristem.
Describe the alternating regions of cambium cells found in the vascular cambium in detail.
- Ray initials: produce radial (like the spokes of a bicycle tire) files of parenchyma cells known as xylem rays and phloem rays. These xylem and phloem rays are living highways of cells for horizontal transport of water and nutrients AND storage of starch and other reserves.
- Fusiform initials: (occur inside a vascular bundle) produce new secondary vascular tissues (usually more xylem), over the years secondary growth produces the wood of a stem (secondary xylem = wood). Anatomically, wood = tracheids, vessel elements, and fibers in angiosperms (cell walls are lignified, dead, at maturity).
- Note - during fusiform/ray initial mitosis one cell stays an initial for future growth and the other differentiates.
Describe the creation of annual rings.
- In temperate climates the cambium is dormant in winter, but reactivated at the beginning of the growing season by Auxin secreted from the shoot tip.
- Spring wood (early wood): 1st xylem to be produced after cambium interruption. The tracheids and vessel elements have a wider diameter and thinner cell walls. This means better water and ion conduction which is needed during the start of the growing season.
- Summer wood (late wood): tracheids and vessel elements have smaller diameter.
Sapwood vs. Heartwood.
- Heartwood: (appears darker) older and no longer functions in water and ion transport (toward center of trunk). Resin material clogs the inside of these tubes and together with the lignified walls provides a central support column for the tree. Resin also helps protect the tree from fungi, bacteria, and wood-boring insects.
- Sapwood: (appears lighter) functions in the upward transport of water and ions.
When is periderm formed? What are the layers of the periderm w/ alt name?
- Periderm replaces the epidermis in plants with secondary growth, begins to form by the end of the first year following the initiation of secondary growth.
- Periderm includes cork cells (phellem), cork cambium (phellogen), parenchyma cells (phelloderm)
- Cork cambium (derived from cells in the cortex) produces cork cells toward the exterior of the cork cambium and phelloderm (living parenchyma cells) to toward the interior.
- Note – cork cells walls accumulate suberin, wax, and lignin which help protect the stem from physical damage, pathogens, and water loss.
- Note – Multiple periderm layers can exist and may slough off.
What are lenticels? Describe their function and how they are formed.
- Areas where the activity of the cork cambium is higher than elsewhere and creates a tear or split.
- They allow for living cells within the trunk to exchange gases with the outside air.
- As the stem increases in size due to the vascular cambium activity, it splits the outer periderm. New cork cambium kicks in and produces a new periderm. The first periderm may keep up with the initial increase in girth for several years by periodic activity of cork cambium which doesn’t necessarily correspond to vascular cambium activity.
Describe “bark” “inner bark” and “outer bark.”
- Bark: all phloem plus periderm (everything to the outside of the vascular cambium)
- Inner bark: secondary phloem
- Outer bark: periderm (cork, cork cambium, living parenchyma)
Which areas of phloem function in a secondary stem? Why?
Only the most recent secondary phloem functions in sugar transport because the rest are destroyed (squished). Older secondary phloem dies and sloughs off as part of the bark as stem diameter increases. Thus, increase in girth is mostly due to secondary xylem accumulation.
Identify and list the function of terminal bud, axillary buds, terminal bud scale scars, bundle scars, leaf scars, lenticels, dilated rays.
- Terminal bud: meristematic area of the shoot, keeps axillary buds in check
- Axillary (lateral) buds: meristematic tissue usually kept inactive by presense of terminal bud
- Terminal bud scale scars: mark the location of previous terminal buds
- Bundle scars: severed ends of vascular bundles (leaf traces) after leaf fall
- Leaf scars: mark where the leaf falls off
- Lenticels: allow gas exchange for living cells in the trunk
- Dilated rays: dilated phloem rays sometimes occur in an attempt to compensate for the rapid girth increase caused by secondary xylem production.
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