2.4 Stems Flashcards
Stems give rise to which TWO types of buds, and what do they do?
Apical bud - surrounds the growing tip of a shoot - (shoot apical meristem)
Axillary bud - forms in angle between leaf and stem - responsible for branching as they develop into side shoots.
Stems - what is a node?
Points at which leaves and axillary buds occur.
Permanent stem between the nodes is called the internode.
Stems - what is the leaf scar?
Once leaves fall off they leave a scar where the leaf joined the stem.
Stems - what are lenticels?
Pores in the cork (woody) that allow gas exchange, allowing cortex to “breathe”.
What is the primary function of a stem
To hold leaves and flowers in optimum positions
Transport between roots and leaves
How does the stem develop?
From germination the stem develops from the Plumule (embryonic shoot)
In seedlings, the stem-like part of the shoot between the cotyledons and first true leaves is called the Epicotyl.
True stem then develops to produce mature leaves and flowers.
Increases in length through cell division and expansion.
Herbaceous growth uses cell expansion to thicken.
Woody growth uses secondary thickening (girth) through the action of the vascular cambium (also in roots).
How do Monocots strengthen?
Secondary growth occurs randomly throughout stem (vascular bundles are scattered throughout ground tissue)
No secondary thickening
Most monocots are herbs so don’t undergo complex processes
Results can be strong, flexible, fibrous stems (think bamboo!) - with outer sheaths
How do Dicots strengthen?
Through secondary thickening (woody growth) - in woody plants!
Mechanical strength - allows for huge, strong growth
During growth, cells of the Vascular Cambium grow and join up to form a central ring - a complete lateral meristem.
This lateral meristem lays down Secondary phloem on the outside and Secondary xylem on the inside
This Secondary Xylem is dead tissues which forces apart the primary xylem and phloem
Cork cambium forms and begins to produce cork - an important part of the bark.
Other functions of stems:
Storage
Climbing
Reproduction
Organisation
Mechanical strength
Photosynthesis
Height
Defence
Reasons for stem HEIGHT
Leaves reach light
Flowers exposed to pollinators
Pollen and seeds dispersed better
Examples of stem STORAGE
Swollen stems of desert plants (cacti / euphorbia)
Underground stem tubers - e.g potatoes
Corms - swollen, tuber-like stem bases - store starch over winter - PERENNATION!
When do stems aid photosynthesis?
When leaves are absent - e.g. succulents
How do stems aid in defence?
Prickles on stems
Stinging hairs
Benefits of climbing stems…?
Plant gains height without having to develop woody tissues
Twining species wrap around other plants for support
What are stems’ roles in reproduction?
Portions of stem can produce clones
Stolons (runners) run above ground and produce roots and shoots
Rhizomes are creeping stems underground and produce new shoots at nodes
Corms produce new corms (cormels) from short stolons underground
How do stems function in water transport
Water moves in the xylem upwards only, by transpirational pull, to form a continuous column of water.
Can happen by way of adhesion of water molecules to xylem vessel wall.
Differences between a woody and herbaceous stem (3):
Woody stems undergo secondary thickening - herbaceous do not
Woody stems develop bark (cork) whereas herbaceous retain the epidermis
Woody stems have a central cylinder of xylem and phloem, whereas herbaceous stems have xylem and phloem arranged in a ring of vascular bundles
Where does starch storage take place in young dicot stem?
In the pith and the cortex
Where does mineral transport take place in stem?
In the xylem
Where does cell division take place in stem?
In the vascular cambium
Differences between monocot and dicot stems (4)
Monocot stems have randomly scattered vascular bundles whereas dicots have them arranged in a ring
Monocot stems are herbaceous and non-woody, whereas dicot stems can be woody and herbaceous
Monocot stems have no vascular cambium, whereas dicot stems do (dicots undergo secondary thickening.
Monocot stems have stomata in epidermis; dicot stems have lenticels in bark.
Stem adaptations:
Rhizome - horizontal stem below the surface - produces new shoots at nodes.
Corm - compressed underground stem - produces new corm above old one.
Stem tuber - underground stem from which new plants arise at “eyes”.
Stolon/runner - horizontal stem which runs along the ground surface and roots at nodes or along its length.
Sucker - vertical stem which arises from roots.
