Organ systems - shoot & root

Shoot system - aerial stems & leaves (vegetative) and sometimes cones or flowers (reproductive parts)

Stems - transport water and nutrients and provide support
- Apical bud or shoot apical meristem (SAM) is where new shoot system cells are produced, allowing plant to grow upwards
- Stems are divided into nodes & internodes
- Nodes are where leaves attach to a stem & where axillary buds develop
- Axillary buds are typically dormant & controlled by terminal bud but also have SAMs and contain potential to develop into a branch
- Internodes are stem areas that elongate to spread the leaves apart and optimize exposure to sunlight and air circulation
Leaves - photosynthesis & gas exchange
- Cells are rich in light-harvesting chlorophyll pigment, predominantly green
- Composed of a flat, thin blade responsible for capturing sunlight and are webbed with veins that act as internal plumbing
- Covered in waxy cuticle to prevent drying
- Have pores called stomata on surface responsible for capturing CO2 and releasing oxygen
- Leaf blades often attached to main stem via thin, sturdy petiole responsible for holding blade upright

Shoot variations

Shape & structure of leaves vary from species of plants depending on their adaptation to climate & available light

Leaves represent heavy investment on the part of the plants bearing them

Leaf arrangement - shoot systems have wide variation in phyllotaxy or leaf arrangement on the stem

Alternate - one leaf at each node, alternating sides
Opposite - two leaves at a node, on opposite sides
Whorled - three or more leaves arranged around same node
Basal - leaves occur in tight rosette at base of plant

Leaf form

Two forms of broad leaves based on the way the leaf blade (also called lamina, widest portion of the leaf) is divided: simple and compound

Simple - undivided blades
Compound - blades that are fully divided into leaflets, seperated along main or secondary vein
- Leaflets may have petiolules and stipels
- Important to recognize where petiole occurs (where leaf attaches to a stem node) to identify compound leaves
- Compound leaves can be pinnately (leaflets on both sides of the petiole) or palmately (radiating from a single point of the petiole) compound

Third form differs from broad leaf types - needles

Very low light absorptive surface area
Well-suited to drier climates by water conservation
Last three to four years, while broad leaves only live for growing season
Evergreens have advantage over broad-leafed deciduous plants since cost of leaf synthesis can be recovered via phosynthesis over several growing seasons

Leaf venation

Veins provide transportation of water and nutrients between leaf and stem and play crucial role in maintenance of leaf water status and photosynthetic capacity

Also play role in mechanical support of the leaf

Angiosperms possess two main types of leaf venation - parallel and reticulate

Parallel (all veins run the same direction), monocots are generally parallel venated
Reticulate (veins form a net-like webbing throughout the leaf) dicots are generally reticulate venated
- Pinnate - single midrib and secondary veins branching off at intervals
- Palmate - primary veins radiate from a single point near the base

Leaf margins

Edges of leaf blades come in many forms - entire, lobed, toothed

Entire - smooth, complete edge
Lobed - indentation of the edge towards the midrib
- Create a smaller leaf since deep division makes it easier for wind to carry away heat and water vapor
Toothed - saw-like edge with small teeth

Leaf shape

Flat shape maximizes contact with surrounding air, promoting cooling

Long leaves help with temperature regulation - good for hot conditions
Thin leaves help prevent water loss and are good for dry conditions
Large, broad leaves can harvest lots of light and are good for gloomy conditions

Long term changes are adaptations

Short term changes are plasticity

Leaves that receive lots of sunlight tend to be smaller to reduce light-absorbing area and tend to have more complex edges and lobes to disperse heat rapidly
Leaves in more shaded environments are usually larger to increase light-absorbing surface area and tend to have reduced expressions of lobes and edges

Leaf surfaces - glabrous to pubescent

Glabrous - smooth; help encourage shedding water in wet environments to prevent interference with light capture or gas exchange
Pubescent - hairy; help insulate the leaf in cold or heat, prevent excessive water loss in arid conditions and provide UV protection under high light

Root system - organs of the plant found below-ground

Can modify environment through growth and chemical interactions with organisms, unlike shoots
Anchor the plant body in place
Responsible for absorption of water and minerals
Storage space for extra food and nutrients
Each root tip contains equivalent to apical bud, root apical meristems (RAM) where new root cells are produced allowing them to grow downwards
Roots only absorb water at their tips through root hairs, rest of the root develops a thick, protective outer layer and transports water form tips to shoots

Root variations

Categories - coarse and fine

Coarse roots provide stability, storage, transport, exhibit branching and sometimes secondary (thickening) growth
Fine roots are most distal part of root system, responsible for water and nutrient uptake due to their highly permeable membranes

Systems - tap and fibrous

Tap roots - single, stout primary root growing from stem base with thinner lateral roots growing from it
- Best for anchorage, reaching deep sources of water, storage of extra food
Fibrous roots - primary root is replaced by a mass of equally sized roots developing from main stem base
- Water and mineral absorption and plant stabilization
- Shallow netting of roots prevents soil erosion
Special roots develop from shoot system called adventitious roots
- Above or underground
- Increase plant stability, or allow plant to reproduce asexually

Architecture

in well-drained uplands, roots grow deep to reach down to the level of rainwater & snowmelt
in waterlogged lowlands, roots stay shallow
In between, high growth rates and drought can send roots many meteres down to the saturated zone just above the groundwater table

	Created by Sophia Wright about 6 years ago

Note on Unit 3: Plant Morphology, created by Sophia Wright on 21/09/2019.