Support a plant….be a stem!

Plant structure: Part II

Stem functions•Support for leaves and flowers

•Transport link to and from leaves, roots and reproductive parts using xylem and phloem

•storing water and nutrients for future use (parenchyma)

•can also help protect plan (eg. Can be spiny like in a cactus or raspberry plant)

Stem types

• Herbaceous plants – stems without wood- Thin epidermis and photosynthetic

• Woody plants – guess what? Stems with wood- thick, hard epidermis (bark) and usually no

photosynthesis - All gymnosperms have woody stems and are dicots

Anatomy of herbaceous stems• Vascular tissue consists of vascular bundles

arranged in the ground tissue• Vascular bundles are long, continuous strands of

xylem and phloem (xylem is always closer to the centre of the stem than the phloem)

• Different arrangement for monocots (e.g corn) and dicots (e.g. dandelion)

• Monocots – vascular bundles are throughout the ground tissue

• Dicots – vascular bundles form a ring

Anatomy of woody stems• Woody plants: Stems contain wood• More complex than herbaceous plant stems• Grow thicker due to vascular cambium, which

is a layer of meristematic (undifferentiated) tissue in the vascular tissue that produces xylem and phloem cells

• Xylem tissue is on the inside of the vascular cambium; phloem on the outside

• ‘Wood’ is actually many layers of xylem cells.

Layers of xylem cells:

Sapwood – younger xylem which transport water and minerals to leaves

Heartwood – older xylem full of resin, rigid to support tree

Other components of woody stems• Bark – all tissues outside of the vascular

cambium including phloem, cork cambium and cork

• Phloem – transports sugars throughout the plant

• Cork cambium – meristematic tissue that produces cork – the outer layer of the tree; prevents water loss

Growth rings

Woody stems have obvious growth rings in temperate regions – growth only in spring and summer

Spring – more xylem cells – thin wall – light coloured wood

Summer – less xylem cells – thicker walls – darker ring

Together – growth ring

Stem specializations

• Not all stems give support to leaves and other parts; some are underground – stolons (spider plants)

• Other stems have formed vines (e.g. Peas)• Tubers – e.g. Potatoes• Rhizomes – e.g. Iris

Getting to the root of it…

Plant Structure: Part III

Function of RootsRoots are responsible for:

•Anchoring the plant to the ground

•Extracting water and minerals from the soil • Storage of food (carbohydrates or sugar)

Typical root parts• 1. Primary root – first root development

from the seed• 2. Secondary roots – smaller root branches

growing sideways from the primary root• 3. Root cap – a protective cap covering the

growing region of the root tip. It is designed to drill the soil and it is able to guide the root growth by perceiving gravity.

• 4. Root hairs - are microscopic extension of the epidermal cells near the tip of a root. They absorb water and nutrients from the soil.

Types of Root Systems

1. Taproot system – large, thick, main root called a taproot; can have lateral roots; all covered with root hairs

Gymnosperms and angiosperms dicots have taproot systems (dandelion)

2. Fibrous root system – many small roots with many lateral roots with root hairs; shallowers

- In angiosperm monocots (grasses)

Structure of roots

• Root cap – protects• Epidermal cells – with fine root hairs – increase area

of absorption• Vascular cylinder (stele) – centre of root and contains

vascular tissues, xylem and phloem, some ground tissue

• Endodermis – surrounds the vascular cylinder, innermost layer of cortex

• Pericycle – between vascular cylinder and endodermis

Root adaptations

• Aerial roots – ivy and epiphyte orchids

• Expanded roots – carrots

• Toxin release – walnut tree roots release toxins into soil