BASIC SCIENCE II: Plant Morphology and Physiology

Plant Morphology, also known as Phyto Morphology, is the study of external/physical structure/form of plants. It is the study of forms and features of different plant organs such as roots, leaves, flowers, seeds and fruits.

Plant Anatomy is the study of the structure of plants… looking at cells, tissues and organs.

Plant physiology is the study of the function of cells, tissues, organs of plants; and the physics/chemistry of these functions. It is the branch of biology concerned with the vital functions of plants such as nutrition, respiration, reproduction, etc.

The area of plant morphology overlaps with plant physiology and ecology.

Plant Anatomy: Cells

• Plant cells are basic building blocks
• Can specialize in form and function
• By working together, forming tissues, they can support each other and survive
• Levels of organization

atoms > molecules > cells > tissues > organs > whole plant

Plant Tissues Types

All plant organs (roots, stems, leaves) are composed of the same tissue types.

There are three types of tissue:

• Dermal – outermost layer
• Vascular – conducting tissue, transport
• Ground – bulk of inner layers

1. Dermal Tissue: 

• Epidermis is the outermost layer of cells
• Like the “skin” of animals
• In stems and leaves, epidermis has cuticle, a waxy layer that prevents water loss.
• Some have trichomes, hairs.
• Root epidermis has root hairs, for water and nutrient absorption

2. Vascular tissue

• Transports water and organic materials (sugars) throughout the plant
• Xylem – transports water and dissolved ions from the root to the stem and leaves.
• Phloem – carries dissolved sugars from leaves to rest of the plant

XYLEM

• Transports water and dissolved minerals
• Tracheids: long, thin tube like structures without perforations at the ends
• Vessel elements: short, wide tubes perforated at the ends (together form a pipe, called vessel).
• Both cells have pits (thin sections) on the walls

Xylem Cells

• Xylem cells are dead!
• They are hollow cells and consist only of cell wall

PHLOEM

• Cells that transport organic materials (sugars)
• Phloem cells are ALIVE! (unlike xylem)
• However, they lack
  nucleus and
  organelles

Phloem: transports sugars

• Phloem composed of cells called sieve tube members (STM)
• Companion cells join sieve tube members, are related, and help to load materials into STM
• End walls of STM have large pores called sieve plates

3. Ground Tissue

• Makes up the bulk of plant organs.
• Functions: Metabolism, storage and support.

Plant Organs

Organs: tissues that act together to serve a specific function

Roots

1. Dermal
2. Vascular
3. Ground

• Stems

1. Dermal
2. Vascular
3. Ground

• Leaves

1. Dermal
2. Vascular
3. Ground

Functions of plant organs

• ROOTS: Anchorage, water/nutrient absorption from soil, storage, water/nutrient transport
• STEMS: Support, water/nutrient transport
• LEAVES: Photosynthesis (food production)

ROOTS: “the hidden half”

Functions of roots:

• Anchorage
• Absorption of water & dissolved minerals
• Storage (surplus sugars, starch)
• Conduction water/nutrients

Root Epidermis

• Outermost, single layer of cells that:

– Protects (from diseases)

– Absorbs water and nutrients

Root Hairs

• tubular extensions of epidermal cells.
• Increase surface area of root, for better water/nutrient absorption

Root Hairs: water and mineral absorption

Root hairs increase surface area for better

absorption.

Meristematic activity, which increases the length of the root, occurs only at the tip. When the cells here divide, they produce the new root cells and root cap cells. The root cap cells are sloughed off as the root grows through the soil. The epidermal cells produce root hairs a short distance above the tip. Root hairs are part of epidermal cells.

Root Cortex

• Stores starch, sugars and other substances (resins, latex, essential oils, and tannins)

Root Ground Tissue

• In roots, ground tissue (a.k.a. cortex) provides support, and often stores sugars and starch (for example: yams, sweet potato, etc.)

Root Cortex: Endodermis

• Endodermis: the innermost layer of the cortex

Root cortex: Casparian strip

• The Casparian strip is a water-impermeable strip of waxy material found in the endodermis (innermost layer of the cortex).
• The Casparian strip helps to control the uptake of minerals into the xylem: they have to go through the cytoplasm of the cell!

STEMS

• Above-ground organs (usually)
• Support leaves and fruits
• Conduct water and sugars throughout plant (xylem and phloem)

Stem anatomy

Dermal, ground and vascular tissues

Types of Stems

Tissues of stems

• Epidermis (Dermal tissue type)
• Provides protection
• Has cuticle (wax) prevents water loss
• Trichomes (hairs) for protection, to release scents, oils, etc.

Stem Vascular tissue

• Vascular bundles – composed of both xylem and phloem
• Xylem

–Conducts water

–Support

• Phloem

–Conducts food

–Support

Vasular Cambium

• Occurs in woody stems
• Vascular cambium located in the middle of the vascular bundle, between xylem and phloem

Vascular tissue: Trees

• Vascular tissue is located on the outer layers of the tree.

Girdling: cutting around a tree

• Damages the phloem and xylem, eventually killing the tree

Vascular tissue forms rings in trees 

• Annual rings: xylem formed by the vascular cambium during one growing season
• One ring = one year

Ground tissue: Cortex & pith

• Stores food (e.g. potato)
• Site of Photosynthesis (when green)
• Support cells

LEAVES

• ‘Photosynthetic factories’ of the plant…
• Function: Photosynthesis – food production for the whole plant
• Blade: Flat expanded area
• Petiole: stalk that connects leaf blade to stem, and transports materials

Leaf Anatomy

• Leaf anatomy is correlated to photosynthesis:

Carbon dioxide + Water  ->   sugars + oxygen

Leaf Epidermis

• Is transparent – so that sun light can go through.
• Waxy cuticle protects against drying out.
• Lower epidermis: stomata with guard cells – for gas exchange (CO2, H2O in; O2 out)

• Trichomes (give fuzzy texture)

Leaf Vascular Tissue

• VEINS -> vascular tissue of leaves.
• Veins are composed of xylem (water transport) phloem (food transport)
• and bundle sheaths, cells surrounding the xylem/phloem for strength & support

Leaf Mesophyll

• Middle of the leaf (meso-phyll)
• Composed of photosynthetic ground cells:
• Palisade parenchyma (long columns below epidermis; have lots chloroplasts for photosynthesis)

Spongy parenchyma (spherical cells with air spaces around, (for gas exchange)

Plant Water Transport

• Water has great cohesive forces (molecules sticking to each other) and adhesive forces (attaching to walls of xylem cells)

Transpiration-cohesion Theory for water transport in the xylem.

• Evaporation of water in the leaves (through stomates) generates the ‘sucking force’ that pulls adjacent water molecules up the leaf surface
• Like a long chain, water molecules pull each other up the column.
• The column goes from roots -> leaves.
• What’s amazing is that the water moves up by using the sun’ evaporative energy.
• Plants control transpiration by opening/closing stomata

Sugar translocation

• Sugars made in leaf mesophyll cells (source) diffuse to phloem cells in the vascular bundles.
• Companion cells load dissolved sugars into the phloem STM using energy (ATP).
• Water moves into cells with high sugar concentration.
• Osmotic water flow generates a high hydraulic pressure that moves dissolved sugars through the phloem to the rest of the plant (sink).

Pressure flow in phloem

• Sugars made in the leaves are loaded into companion cells and into phloem STM.
• Water (from xylem) moves in by osmosis, creating pressure flow down the phloem.

Plant Hormones

• Chemical compounds produced by plants
• Effective at very low concentrations
• Five major hormone groups are:

1. Auxins
2. Gibberellins
3. Cytokinins
4. Abscisic Acid
5. Ethylene

1. AUXINS

• Promote cell growth
• Involved in gravitropism and phototropism
• Control fruit development

2. GIBBERELLINS

• Promote stem elongation

3. CYTOKININS

• Promote cell division and organ differentiation

4. ABISCINIC ACID

• Promotes seed dormancy
• Causes stomata closing

5. ETHYLENE

• Gaseous hormone, very simple formula (C2H4)
• Ethylene promotes fruit ripening

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