2.4 Stems

Question 1

Stems give rise to which TWO types of buds, and what do they do?

Answer 1

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.

Question 2

Stems - what is a node?

Answer 2

Points at which leaves and axillary buds occur.

Permanent stem between the nodes is called the internode.

Question 3

Stems - what is the leaf scar?

Answer 3

Once leaves fall off they leave a scar where the leaf joined the stem.

Question 4

Stems - what are lenticels?

Answer 4

Pores in the cork (woody) that allow gas exchange, allowing cortex to “breathe”.

Question 5

What is the primary function of a stem

Answer 5

To hold leaves and flowers in optimum positions

Transport between roots and leaves

Question 6

How does the stem develop?

Answer 6

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).

Question 7

How do Monocots strengthen?

Answer 7

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

Question 8

How do Dicots strengthen?

Answer 8

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.

Question 9

Other functions of stems:

Answer 9

Storage

Climbing

Reproduction

Organisation

Mechanical strength

Photosynthesis
Height

Defence

Question 10

Reasons for stem HEIGHT

Answer 10

Leaves reach light

Flowers exposed to pollinators

Pollen and seeds dispersed better

Question 11

Examples of stem STORAGE

Answer 11

Swollen stems of desert plants (cacti / euphorbia)

Underground stem tubers - e.g potatoes

Corms - swollen, tuber-like stem bases - store starch over winter - PERENNATION!

Question 12

When do stems aid photosynthesis?

Answer 12

When leaves are absent - e.g. succulents

Question 13

How do stems aid in defence?

Answer 13

Prickles on stems

Stinging hairs

Question 14

Benefits of climbing stems…?

Answer 14

Plant gains height without having to develop woody tissues

Twining species wrap around other plants for support

Question 15

What are stems’ roles in reproduction?

Answer 15

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

Question 16

How do stems function in water transport

Answer 16

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.

Question 17

Differences between a woody and herbaceous stem (3):

Answer 17

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

Question 18

Where does starch storage take place in young dicot stem?

Answer 18

In the pith and the cortex

Question 19

Where does mineral transport take place in stem?

Answer 19

In the xylem

Question 20

Where does cell division take place in stem?

Answer 20

In the vascular cambium

Question 21

Differences between monocot and dicot stems (4)

Answer 21

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.

Question 22

Stem adaptations:

Answer 22

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.