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Introduction to Stems

Objectives

  • Describe the difference between monocot and dicot stems.
  • List the secondary tissues in a dicot stem.
  • Explain the formation of an annual ring.
  • Explain the pressure-flow hypothesis, and tell what substances are transported in phloem.
  • Describe the transpiration-cohesion theory.

Functions of Stems

  • Transport of food and water
  • Photosynthesis
  • Support of leaves, flowers, and fruit
  • Water Storage (fleshy stems of succulents, cacti)
  • Reproduction (potato eyes, strawberry runner, layering)
  • Food storage (in parenchyma cells)

Types of Stems
Monocot
Dicot

 

 

 

 

 

 

 

 

Herbaceous
Woody
Herbaceous
Woody

Modified Stems

Name

Description

Example

Rhizome

thick, fleshy horizontal underground stems. Aerial shoots form at the nodes

lily of the valley, iris

Stolon

similar to a rhizome but more slender growing just under the ground.

crabgrass

...Runner

term given to a stolon that lies on the surface of the ground

strawberry runner

...Tuber

fleshy swollen end of a stolon used for food storage.(The eyes of a potato are buds developing at the node)

potato

Bulb

short basal stem with circular layers of thick scale leaves.

onion, lily, daffodil, tulip, hyacinth

Corm

short thick underground stem with thin scale leaves.

gladiola, crocus

Primary and Secondary Growth in Stems

Origin

Tissue

Monocots

Dicots

Gymnosperm

Dermal Tissue

Epidermis

Present

Present

Present

Ground Tissue

Cortex

Pith

Not well defined

Not well defined

Well defined

Well defined

Well defined

Well defined

Vascular Tissue

Vascular bundles

Scattered Arrangement

Ordered Arrangement

Ordered Arrangement

1. Primary growth is produced by the apical meristem and results in increased length.

2. Secondary growth is due to lateral meristem tissues called cambium and results in increased circumference (diameter)

 

Vascular cambium produces secondary phloem SECONDARY

on the outside and secondary xylem on the inside. PHLOEM

 

 

Cork cambium produces cork which matures into bark. VASCULAR CAMBIUM

 

 

SECONDARY XYLEM

Comparison of herbaceous monocot and dicot stems

A. Pith

B. Cortex

C. Vascular bundles

D. Vascular cambium

E. Xylem

F. Phloem

G. Epidermis

H. Mechanical tissue

I. Air space

 

*Bundle Arrangement

Monocot
Dicot

*Vascular bundles in divots are organized in a circle.

*Vascular bundles in monocots are scattered.

 

Water Transport in Stems

Definitions:

1. Transpiration is the evaporation of water through stromata.

2. Transpiration-cohesive theory - water may move up through a plant because it is cohesive.

3. Guttation is the process by which water taken up by the plant roots forces water out of the leaves.

Guttation

1. Water is "forced" out of the leaves by root pressure forming water droplets at the margins of the leaves.

2. Usually occurs at night when the humidity is to high for transpiration.

3. Very characteristic of strawberry plants.

 

Transpiration-Cohesion Theory

1. Due to the cohesive nature of water molecules there is a continuous column of water in the xylem cells (vessels and tracheids) from the roots to the leaves.

2. Water evaporates through the leaves through stomates (transpiration) producing a low pressure.

3. Water moves from high pressure areas (in the roots) to low pressure reas at the leaves.

4. As the leaves loss water by evaporation , called transpiration, water from adjacent cells replace it., thus the entire column of water is moved upward.

5. The speed of transpiration changes during the day. It is greatest at noon. Trunks actually shrink in diameter during the daytime.

 

Secondary Growth in Woody Dicots

1. Vascular cambium develops between the primary xylem and the primary phloem in the vascular bundles of divots. Eventually the cambium forms a circle.

2. Vascular cambium produces secondary xylem on its inside and secondary phloem on its outside.

3. Differences in the seasonal growth of the secondary xylem and phloem produces an annual ring which can be used to age trees.

4. Wood contains parenchyma cells called rays that form flat tissue radiating out from the center of the trunk. Rays function to store food and transport food and water laterally.

5. Cork cambium develops in the area outside the phloem. It produces cork which replaces the epidermis and cortex .

Woody Dicot Structure 1

A. Vascular cambium

B. Cork cambium

C. Cortex

D. Rays

E. Cork

F. Secondary phloem

G. Secondary xylem

H. Annual ring

 

Secondary Growth in Woody Dicots 2

1. Vascular cambium forms new secondary xylem cells.

When water is plentiful in the Spring the cells are wide with thin walls - Springwood

When water is less plentiful, it produces smaller thick celled walls called - Summerwood

The visible difference between summerwood and springwood produces annual rings.

2. The oldest xylem cells are found toward the center of the stem. They are the first to become clogged and take on a dark in color with age. This darker colored wood is referred to as heartwood.

3. The xylem cells than are newer (toward the outside) are lighter in color and referred to as sapwood.

4. Bark is the protective covering of the outside of woody plants. It consists of cork, cork cambium, cortex and secondary xylem.

5. Secondary phloem produced on the outside edge of the vascular cambium is part of the bark.

6. Cork cambium produces cork towards the outside. Cork cells die quickly and because they cannot expand they split producing characteristics barks of different trees.

 

Pressure Flow Hypothesis

Definitions

1. Osmosis - the movement of water molecules from areas of high concentration to areas of low concentration through a semipermeable membrane.

2. Active Transport - the movement of a substance across a membrane involving the use of energy.

 

 

Transport of Sugars

1. Sugars are the food that moves in the phloem.

2. Movement of the sugar from source (leaves) to the sink (roots) for storage is explained by the pressure-flow hypothesis.

3. Sugar is pumped by active transport into the sieve tubes at the source (leaves).

4. Water follows the sugar due to osmosis creating "pressure".

5. Sugar is pumped out of the sieve tubes at the sink (roots). Water follows due to osmosis

6. The pressure difference between the source and the sink causes the "flow" of food (sugar) down

 

External Structure of a Woody Dicot Stem

Description/Function

Term

Place on a stem where leaves develop.

Node

Space between two adjacent nodes

Internode

Places where bud scales surrounded the leaves

Bud Scale Scar

Marks left on a stem after a leaf falls off.

Leaf Scar

Circular scars inside the leaf scar where the vascular tissue left the stem and entered the leaf stem.

Vascular Bundle Scar

Buds that develop on the side of a stem.

Axillary or Lateral Bud

Buds at the tips of branches. They increase stem length.

Termnal Bud

Modified leaves that protect the buds during winter.

Bud Scale

Porous regions in the stem used for gas exchange.

Lenticles

Protected, dormant tissue that develops into new stems, leaves, or flowers.

Bud

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