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What are the functions of roots?
Anchorage, absorption & conduction (minerals/water), storage (energy/nutrients), synthesis of hormones
Describe the various root systems in depth.
- Taproot system: system established by the primary root and its lateral branches of roots. Tend to penetrate deep into the soil. In gymnosperms and eudicots the primary root grows downward giving rise to many lateral roots (not monocots). Penetrate deeper than fibrous roots.
- Fibrous root system: have a short-lived primary root. Instead, many adventitious roots branch off of the stem. This system does not have any one prominent root, but is a tangled mass of roots. Better ground cover than taproots.
- Certain root systems can be a blend of both.
What factors affect root system depth?
Soil composition, temperature, and moisture.
Where does most absorption of water/ions occur in the soil, and by what?
Feeder roots absorb most water/ions for the root system in the upper 15cm of soil. (~6 in)
What is the average ratio between the spread of the roots and the spread of the tree crown?
The spread of the roots is typically 4-7 times greater than the spread of the tree crown.
Describe the root cap and its functions in detail.
Special area of living cells (parenchyma) at the distal tip of the root that functions to physically protect the apical meristem (like a helmet). Also secretes mucigel (a polysaccharide slime) which lubricates the soil around the grown root tip and gives shelter to helpful bacteria.
Describe and give the functions of the various zones that function in primary growth in detail.
- Zone of cell division: apical meristem and its derivatives. New cells are added in this area. Just above the apical meristem the products of cell division form the three cylinders of primary meristems (protoderm, ground meristem, and procambium). If you can see mitotic cells the tissue MUST BE meristematic.
- Zone of elongation: Cells enlarge (elongate) in this area, which is the driving force behind pushing the root tip through the soil.
- Zone of maturation: The three primary meristems differentiate into their respective tissues in this zone. It can be easily recognized by the appearance of root hairs.
Describe the functions and anatomy of root hairs in detail.
- Literal extensions of the epidermis, root hairs begin to form in the zone of maturation. They are extremely compact and dramatically increase the surface area for water absorption by the plant.
- Their appearance can be used to identify the zone of maturation.
Trace the sequence of water/ions making it from soil to the xylem of the root.
Root hairs -> epidermis -> exodermis -> endodermis -> vascular cylinder -> vessel elements
Describe the endodermis and exodermis in detail.
- Endodermis: innermost layer of cells of the cortex. Cells contain a casparian strip (wall thickening made of suberin and lignin) which creates a impermeable layer for water. This helps to regulate the passage of materials into the vascular cylinder (like a filter) by forcing the material to go THROUGH the cells, and not between them.
- Exodermis: outermost layer of cells of the cortex. Anatomically similar to the endodermis, but functions to retain water within the root.
- Both the endodermis and exodermis are layers of the cortex because they are formed from the ground meristem.
What is the vascular cylinder?
The vascular cylinder is an area of the root during primary growth that contains the vascular tissues xylem and phloem and a special single layered group of cells called the pericycle.
Describe the pericycle in detail.
- The outer-most layer of cells in the vascular cylinder, located just interior of the endodermis. This group of cells is meristematic and is the source of lateral roots (roots that branch from the primary root).
- The pericycle is critical to the process of secondary growth (horizontal growth).
What is the difference between monocots and dicots? How do their root cross-sections differ during primary growth?
- Monocot: smaller grass-like plants (wheat, barley, etc). These roots have a pith in the middle. Tend to undergo very little secondary growth in roots.
- Dicots: all other plants. These roots lack a pith, and have a small vascular bundle in the center which houses the xylem, phloem, and pericycle. Tend to undergo secondary growth in roots.
Describe secondary growth in detail (cells, root changes, etc).
- Vascular cambium and cork cambium are the main constituents of secondary growth and both ultimately came from the procambium.
- Vascular cambium: produces secondary xylem toward the inside and secondary phloem toward the outside.
- Cork cambium: produces the periderm. Phelloderm (parenchyma cells) toward the inside and cork toward the outside.
- Gain of width is due to an accumulation of secondary xylem (hard secondary walls don’t shrink under pressure, causing expansion).
- Primary phloem (except fibers) gets crushed as secondary phloem is added (soft/squishy primary wall only).
- Secondary growth causes both the epidermis and cortex (including endodermis) to be sloughed off.
Describe the various root modifications in detail.
- Aerial roots: adventitious roots that can serve a variety of functions (aerial roots of orchid can photosynthesize, prop roots of corn act as a supporter of the plant).
- Pneumatophores: exhibit negative gravitropism (grow against gravity)
- Fleshy roots: storage parenchyma permeate the vascular tissue for storage of nutrients (carrot, potato, etc)
- Water storage root: Storage of water in roots (manroot)
- Adventitious buds: Some roots can produce aerial stems (suckers). These can become separated by the roots and develop from the sucker. (essentially they develop in alternate locations than would be expected).
- Parasitic roots: (dodders) develop haustoria (projections) that invade the phloem and xylem of a different plant. These plants are not green and cannot carry out photosynthesis on their own, they are true parasites.
What are the functions of the stem?
- Support (leaves) and transport
- Xylem: moves water and ions (inorganic) from roots to leaves
- Phloem: moves organic compounds from leaves to roots
Describe the basic development of a nonwoody stem, and what is responsible for its growth.
- Apical meristem adds cells that become the primary meristems (which further differentiate into the tissues)
- Apical meristem adds leaf primordial (will become leaves) and bud primordial (will become axillary buds then lateral stems and leaves)
- Phytomeres: repeated units added by the Apical Mersitem that consist of a node and its leaf, the internode below the node, and the inferior bud
Define node, internode
- Node: point on the stem where the leaves attach
- Internode: the distance between nodes (stem height increase is due to enlargement of cells within internodes)
Describe the arrangements of stem (cross section) in detail w/ visual descriptions and examples.
- Dicots: Vascular bundles arranged in a ring around a pith with pith rays (elderberry, medicago)
- Monocots: Scattered vascular bundles within ground tissue with no discernable cortex or pith
- Herbaceous dicots have further variability in that they can have room for a small amount of secondary growth (medicago) or be completely unable to have secondary growth.
- Vascular bundles of dicots that cannot perform secondary growth look like the “curious George face” with a bundle sheath – a layer of sclerenchyma cells – surrounding them (closed bundle)
- Vascular bundles of dicots that can perform little secondary growth do not contain a bundle sheath, and have a layer of vascular cambium separating the xylem and phloem (open bundle)
Briefly describe the characteristics of monocot and dicot roots and stems.
- Monocot root: vascular cylinder contains a pith, vascular bundles encircle the outer perimeter of the vascular bundle.
- Monocot stem: vascular bundles are located sporadically throughout the cross-section
- Dicot root: vascular cylinder is small and in the center, has xylem in the middle and phloem toward the outside
- Dicot stem: (medicago) vascular bundles encircle a pith, pith rays connect to the outer cortex.
Describe the leaf and stem vascular tissue connection points.
- Vascular tissue connects the leaves to the stem very early on as the procambial strands develop.
- Leaf traces: near the node, vascular tissue from the stem diverges toward the leaf (just the point of divergence)
- Leaf trace gaps: Above these diverging points there are wide gaps of ground tissue
Describe the basic anatomy of the leaf including exceptions, and the different categories of leaves.
- Commonly has lamina (blade) and petiole (stalk). May contain a stipule; a spiky structure at the base of the petiole.
- Sessile leaves: leave without a petiole. The base of the leaf expands into a sheath that encircles the stem. Common in grasses.
- Simple leaf: one large blade connected to a petiole.
- Compound leaf: rather than a large blade there are several leaflets with their own petioles. An axillary bud Is located at the base of the entire blade allowing you to discern between a single leaf or stem. (Also important for determining simple vs compound leaf).
- Pinnately Compound leaf: leaves arise from either side of central rachis (an extension of the petiole) and look similar to a feather
- Palmately compound leaf: no rachis; leaflets branch out from the petiole (like a hand at the end of an arm).
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