Plant Histology and Growth

Plant Histology

• Plant tissue is divided into simple and complex types
• Simple tissue consists of one cell type, such as parenchyma, collenchyma, or sclerenchyma
• Complex tissue consists of more than one cell type like xylem and phloem
• Plant tissue is classified as permanent or meristematic
• Permanent tissue is mature and incapable of cell division
• Meristematic tissue has undifferentiated cells that actively divide by mitosis

Plant Growth

• Growth in animals is determinate; there is no growth in adults
• Growth in stems and roots is indeterminate, generated at meristems undergoing cell division and expansion
• Meristematic cells divide to produce new cells indefinitely
• One cell remains unspecialized and retains the ability to divide
• The other cell differentiates and specializes to perform a specific role

Meristems in Vascular Plants

• There are three types of meristems: apical, lateral, and intercalary
• Apical meristems are at the tips of roots, stems, and buds
• Lateral meristems include the vascular cambium and cork cambium
• Intercalary meristems occur between mature tissues
• Primary growth occurs through mitotic cell division of apical meristem cells followed by differentiation, responsible for increase in length
• Secondary growth occurs through mitotic cell division of lateral meristem cells which differentiates the daughter cells, responsible for growth in diameter.
• Secondary produces wood and bark

Cork Cambium

• Cork cambium produces cork cells, which replace the epidermis
• Cork cells secrete suberin (a waxy substance) and then die

Ground Tissue System: Parenchyma

• Composed of unspecialized cells acting as packing tissue
• Most common cell type with large vacuoles, thin primary cell walls, and a shared middle lamella
• Cytoplasm is pushed to the periphery
• Cell shape is variable, including rounded, lobed, flattened, polyhedral, or elongated forms
• Metabolically active, used in vital activities
• Some cells carry out photosynthesis
• Intercellular air spaces are usually prominent, supporting gas exchange
• Found in bulk in organs like stems and roots, such as pith and cortex
• Aqueous parenchyma (modified) is responsible for water storage in succulent plants and halophytes
• Aerenchyma (modified parenchyma) is present in aquatic plants
• Parenchyma also exists in endodermis and pericycle of roots, and among xylem vessels and phloem cells
• Mesophyll (palisade and spongy layers) is a modified parenchyma, chlorenchyma with chloroplasts.
• Lenticels exists

Collenchyma

• Collenchyma cells are generally elongated
• Primary walls are characteristically thick at the corners due to pectin deposition, but no secondary walls
• Collenchyma is a flexible tissue that supports stems, petioles, and growing organs without snapping
• Tissue adjusts to stress by depositing more cellulose
• Lacks lignin for growth and stretching
• Collenchyma is located in the hypodermis/outer cortex of dicot stems, in bundles forming ridges, also in dicot leaves, providing support for the vascular bundles

Sclerenchyma

• Cells have thickened secondary walls for support conferring firmness or rigidity
• Sclerenchyma cells undergo programmed cell death after lignifying their cell walls
• Fibers relatively rigid support to wood and bark of trees
• Sclereids pack together densely, as in nut shells or seed coats
• Primary walls are heavily thickened with lignin deposits
• These deposits provide high tensile and compressional strength and make walls hydrophobic and impermeable
• Fibers and sclereids have simple pits in their walls where lignin is not deposited due to plasmodesmata
• Common in cortex, pith, phloem, fruit, and seeds
• Found densely packed in nut shells, seed coats, and stone fruits
• also in xylem and phloem and bordering vascular bundles
• Hypodermis of some monocots

Dermal Tissue System: Epidermis

• A one-cell-thick layer covering the primary plant body providing a protective layer
• Guard cells, trichomes, and root hairs are specialized cells found within the epidermis
• The cuticle is absent in roots however a thicker cuticle will occur in plants in dry climates.
• May be ruptured and replaced by a cork layer during secondary growth
• Monocot leaves have elongated cells with parallel arrangements of stomata and guard cells
• Dicot leaves have cells with wavy margins and irregularly arranged stomata

Guard Cells

• Pairs of cells with a pore (stoma) in between
• Unevenly thickened cell walls lead to the opening of stomatal pores: unequal stretching of the inner and outer walls lead to opening of stomatal pore
• Only epidermal cells containing chloroplasts
• Control of stomatal aperture allows gas exchange and control of transpiration rate

Trichomes/Hairs

• Outgrowths from the epidermis
• Occur on stems or aerial parts of a plant
• Hooked hairs prevent stems from slipping from supports
• Glandular trichomes excrete excess salt from salty soils, preventing toxic accumulation
• Some hairs retain moisture
• May secrete scents or enzymes
• Protect from herbivores
• Root hairs are unicellular outgrowths of root epidermal cells

Vascular Tissue System: The Xylem

• Two main functions: conduction of water/salts and support
• Lignified cell walls, lignin prevents collapse of cell, critical for support
• Xylem in roots withstands tugging strains of aerial parts during bending
• Four cell types: tracheids, vessel elements, parenchyma, and sclerenchyma fibers

Tracheids

• Single cells that are not fused together like vessel element
• Elongated with tapering end walls that overlap adding to the combined mechanical strength
• Contain perforations of pits, which allows for water and salts to pass between cells
• Dead with empty lumen when mature
• Lignification varies
• Protoxylem is formed early, is flexible and stretchable allowing elongation, near center of stem or root
• Metaxylem is formed later, is rigid, does not stretch and is found near protoxylem wider lumen an possess fibres
• Secondary xylem makes up the vascular cambium
  • Contributes to growth and girth
  • Woody plants and rings of trees

Xylem Vessels

• Typical conducting units of angiosperm xylem (angiosperms possess both tracheids and vessels).
• Long and tubular structures formed via fusion of several cells
• Highly lignified and possess perforated walls
• Dead at maturity
• Supportive
• Vessel element shorter and wider with ends open unlike tracheids, which are closed

Vascular Tissue: Phloem

• Living cells modified for translocation of organic solutes
• Five cell types: sieve tube elements, companion cells, parenchyma (only in dicots), sclerenchyma fibers, and sclereids
• Sieve tubes have long tube-like structures also cellulose and pectic substances in their walls
• These are formed via the end-to-end fusion of cells aka sieve tube elements
• Sieve tubes lack nuclei but are are alive and depend on adjacent companion cells and contain sieve plate aka perforation

Companion Cells

Have dense cytoplasm, supply sieve tube members with ATP and proteins and therefore perform alongside the sieve tube element.

• Sieve tube elements and companion cells are interdependent: If one cell dies, the other dies.
• Sieve tube element + companion cell = 1 functional unit

Phloem vs Xylem

• Sieve tube elements are living cells, not xylem vessels
• No mechanical function/no lignin in phloem
• Phloem parenchyma only exists in dicots -Phloem fibers are similar to sclerenchyma fibers reinforce with with lignin and are important for gaseous exchange through intercellular spaces

Monocots vs Dicots

• Phloem fibers are similar to sclerenchyma fibers i.e. needle shaped with lignin deposition and simple pits; their role is reinforcement
• Parenchyma is used for storage, distasteful tannins or crystals which have roles in herbivore defense or gaseous exchange