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Tissues and the Primary Growth of Stems (Parenchyma (Parenchyma cells have…
Tissues and the Primary Growth of Stems
Concepts
Basic parts of an herb
Leaves
Absorb moisture
Derive minerals from wind-blown dust
Stems
Function primarily as transport and support structures
Produce and hold leaves in sunlight
During winter, they store sugars and other nutrients
May also be a means of survival
Can be used by many species as a means of dispersal
All flowering plants have stems
Division Magnoliophyta also known as angiosperms
Consists of approx. 297,000 species and is the largest division in the plant kingdom
There are nodes where leaves are attached, and internodes, the region between the nodes
Roots
Plants in the bromeliad family are nearly rootless
Definition of an herb
A plant that never becomes woody and covered with bark
1st land plant evolved about 420 million years ago
Had no roots, stems, or leaves
Shores became crowed with these plants
Mutations occurred that allowed vertical growth towards sunshine
Later on specialized tissues and organs began to evovle
Two fundamental types of plant bodies
Herbaceous body, also called a primary plant body
A woody body, known as a secondary plant body
Parenchyma
Parenchyma cells have only primary walls that remain thin
Chlorenchyma cells
Parenchyma cells involved in photosynthesis
Have numerous chloroplasts
Thin wall allows light and carbon dioxide to pass through the chloroplasts
Transfer cells
Parenchyma cells that mediate short-distance transport of material by means of a large, extensive plasma membrane capable of holding numerous molecular pumps
Increase their surface area by having extensive knobs, ridges, and other ingrowths on the inner surface of their walls
Parenchyma tissue
A mass of parenchyma cells. This is the most common type of cell and tissue, constituting all soft parts of a plant
Phloem
Conducts nutrients over long distances
Soft leaves, petals, fruits, and seeds are composed almost completely of parenchyma
Glandular cells
Glandular cells secrete that secrete nectar, fragrances, mucilage, resins, and oils are also parenchyma cells
Typically contain few chloroplasts but have elevated amounts of dictyosomes and endoplasmic reticulum
They transport large quantities of sugar and minerals into themselves
Transform them metabolically
Transport the product out
Collenchyma
Collenchyma cells
Have a primary wall that remains thin in some areas but becomes thickened in other area, most often in the corners
The wall exhibits plasticity
The cell walls require more glucose for their production because of the wall thickness
Sclerenchyma
Has both a primary wall and a thick secondary wall that is almost always lignified
These walls are elastic
The walls are strong enough to prevent protoplast from expanding
Sclerenchyma cells develop from parenchyma cells in mature organs after they have stopped growing and have achieved their proper size and shape
Two types of Sclerenchyma cells
Conducting and Mechanical Sclerenchyma
Subdivided into long fibers and short sclereids
Fibers are long and flexible
Sclereids are short and more or less isodiametric
Transports water and is one of the types of vascular cells
Supports the plant by its strength alone
Areas of low depression in the developing secondary wall, but as wall deposition continues, these areas become narrow pits
Two pits make up a pit-pair
The set of primary walls and middle lamella between them constitutes a pit membrane
Epidermis
The outermost surface of an herbaceous stem
Which is a single layer of living parenchyma cells
Human epidermis is much thicker but consists of dead skin cells
Preventing loss of water to air is a critical function for the epidermis of land plants
Outer tangential walls are encrusted with cutin
A fatty substance that makes the wall impermeable to water
Cutin builds up as a more or less pure layer called the cuticle
Protoderm refers to epidermal cells that are in the early stages of differentiation
Cortex
Interior to the epidermis
In many plants, it is quite simple and homogeneous
composed of photosynthetic parenchyma and sometimes collenchyma
In other species it can be very complex
Containing many specialized cells that secrete latex, mucilage, or pitch (resin)
Cortex cells of most plants fit compactly
Young cells of pith and cortex are known as ground meristem
Vascular Tissues
Two types of vascular tissues that occur in plants
Xylem
Conducts water and minerals
Two types of conducting cells
Tracheids
Vessel elements
Tracheary element
Refers to either type of celll
Phloem
Distributes sugars and minerals
Two types of conducting cells
Sieve cells
Sieve tube members
Sieve element refers to either of the two
They are parenchyma cells, have only primary walls, and must remain alive to conduct
Their plasmodesmata enlarge to a diameter of more than 1 um and are called sieve pores
Sieve pores also occur clustered together in groups called sieve areas
Their nuclei degenerate but the cell remains alive
Stacked end to end with their large sieve areas aligned forming a sieve tube
The end-wall sieve areas with large pores are sieve plates
Vascular system
Plant vascular system in not a circulatory system
Xylem and phloem occur together as vascular bundles
Are arranged in one ring surrounding the pith
Pith is a region of parenchyma similar to the cortex
All vascular bundles are collateral
Each contains both xylem and phloem strand running parallel to each other
The xylem of a vascular bundle is the primary xylem
It is a part of the primary plant body
The vascular bundle phloem is primary phloem
The exterior cells mature as protophloem and cells closes to the metaxylem become metaphloem
Young cells of xylem and phloem are known as provascular tissues
Stem growth and Differentiation
Stems longer by creating new cells at their tips, in regions known as shoot apical meristems
The region just below the apical meristem is the subapical meristem
Its cells are also dividing and growing, producing cells for the region below
