Plant Tissue Types

• Plants consist of meristematic and permanent tissues

Plant Tissues

• Meristematic tissue actively divides through mitosis
• Permanent tissues are matured, and have experienced or are undergoing differentiation
• Three types of permanent tissues:
  • Epidermal
  • Ground
  • Vascular tissues

Epidermal Tissue

• Outermost surface layer of stems, leaves, and roots of young plants
• Epidermal cell walls exposed to air have a waxy and waterproof cuticle layer
• The cuticle aids to reduce water loss through evaporation, protects against mechanical injuries, and pathogens
• Modified epidermal cells:
  • Guard cells control stomata openings
  • Root hair cells increase water and mineral salt absorption surface area for the root

Ground Tissue

• Parenchyma tissues are the simplest living cells that don't undergo differentiation, and have the thinnest cell walls
  • Maintaining turgidity to support herbaceous plants
  • Photosynthesis
  • Storage for starch and sugar
  • Gaseous exchange
  • Repair and regeneration

Collenchyma Tissue

• Living cells that are flexible and have cell walls made of pectin and hemicellulose
  • Cell walls thicker than parenchyma tissues
  • Provides mechanical support and elasticity

Sclerenchyma Tissue

• Consists of dead cells when matured with the thickest cell walls among the ground tissues
  • Supports mechanical strength
  • Helps transport water and nutrients

Vascular Tissue

• Xylem is composed of dead cells without cytoplasm, has lignin in cell walls and elongated, hollow vessels
  • Transports water/mineral salts from roots

Phloem

• Composed of companion cells and sieve tubes, which are living cells having cytoplasm
  • Sieve tubes lack nucleus/ribosome because they decompose
  • Sugars from photosynthesis get transported from the leaves to storage organs

Meristematic tissues

• Undifferentiated living tissues in plants which are responsible for plant growth
• Apical meristems are at the tips of shoots and roots
• Lateral meristem tissues consist of vascular cambium and cork cambium

Zone of Cell Growth

• Shoot and root tips have three cell growth zones
  • Cell division
  • Cell elongation
  • Cell differentiation
• Leads to primary growth for plants

Zone of Cell Division

• Occurs at apical meristems and actively dividing meristem cells through mitosis
• An increase in the number of cells, causing the elongation of the plant stem
• Forms new cells, the cells formed previously get pushed into the zone of cell elongation

Zone of Cell Elongation

• Consist of cells with increasing size through water diffusion by osmosis, where nutrients get absorbed and stored in vacuoles
• Some vacuoles combine to form a large vacuole, vacuolation occurs
• Diffusion of water exerts pressure against the cell walls pushing, elongating, and widening them

Zone of Cell Differentiation

• Consists of differentiating cells that differentiate once they have reached their maximum size
• Cells differentiate to form permanent tissues like the epidermis, cortex, xylem, and phloem
• Epidermal cells in leaves differentiate and form guard cells to control the opening of stomata.
• Epidermal cells in roots differentiate and form root hair cells

Primary and Secondary Growth

• Two types of growth occurring in plants
• Primary growth occurs after germination providing plant with initial structure
• Secondary growth occurs mainly in eudicots and some monocots, increasing the circumference or diameter of the plant stem and root

Primary Growth

• Occurs when the meristem cells in the zone of cell division divide, followed by elongation and differentiation
• Leaf and shoot primordia grow at the shoot tips to form new leaves and shoots, allowing the plant to increase in height
• Cells of the root cap are replaced by meristem cells when exhausted during soil penetration

Secondary Growth

• Caused by the division of the lateral meristem cells, which consist of vascular cambium and cork cambium located in the stem and root
• Cells in cambium ring divide inward forming new xylem and outward to form new phloem

Vascular cambium

• Actively divides and combines to complete a ring
• The cells in the cambium ring divide inwards to form secondary xylem and outwards to form secondary phloem

Secondary Xylem and Pholem Formation

• The cells in the cambium ring divide inwards to form new xylem and outwards to form new phloem
• New xylem tissues are secondary xylem, and the new phloem tissues are secondary phloem
• When undergoing secondary growth, primary xylem is pushed towards the pith, and primary pholem is pushed towards the epidermis

Eudicots and Secondary Growth

• Adding the secondary xylem layer compresses existing layers, causes circumference to increase allowing the epidermis to stretch and crack
• Cork cambium actively divides, forming cork cells on the outer side and replacing the cortex on the inner side

Growth Rings

• Secondary growth happens at different rates and annual rings can appear showing plant age

Necessity of Primary growth:

• Maximizes elongation for photosynthesis.
• Primary phloem transports products from photosynthesis to other parts of the plant
• Transports water and mineral salts via roots, provides support to young plants

Necessity of Secondary Growth :

• Provides plant stability, by increasing stem/root diameters based on plant height
• Mechanical support
• Produces more xylem/pholem, continuously replace old and damaged tissues
• Stronger, thicker bark to protect from injuries or loss of water and pathogen infections
• Increases chances of seed production and enhances lifespan

Plant Lifespan Growth Types

• Annual
• Biennial
• Perennial

Annual Plants

• Have a one-year lifespan
• Completes biological cycle from germination to the production of seeds

Biennial Plants

• Two-year lifespan with two growth seasons. The first season consists of vegetative growth and the second season consist of reproduction
• Briefly stop the growth process during winter then during spring and summer growth continues

Perennial plants

• Live for two years or more, with lifespan dependent on conditions
• Undergoes continuous changes, and is made up of woody perennial plants and herbaceous perennial plants

Growth curve of annual plants

• This is a sigmoid curve with four stages
  • Stage A: Decreasing dry mass involves the food in the cotyledon getting used up prior to photosynthesis
  • Stage B: Rapid dry mass increase because of photosynthesis
  • Stage C: Constant dry mass where the growth rate is zero because the plant is matured
  • Stage D: Decreasing dry mass caused by aging, lower rates of photosynthesis, and the plant shedding material

Growth curve of biennial plants

• Involves combined two sigmoid curves
• First growth season consists of plants leaving and food is stored in tubers.
• Second growth season, the food stored in the tubers is used to produce flowers and seeds

Growth curve of perennial plants

• A series of small sigmoid curves.
• Growth happens during spring and Summer with decreasing growth and high light intensity during winter