Transcript Stems - SBI3USpring2014

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 Roots
Roots 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