Stems

Goals

1. To understand how stems are put together

2. To understand how stems grow

3. To become familiar with the processes that allow movement of fluids up xylem and down phloem

Outline

Stem structure

External morphology

• internodes and nodes
• lenticels
• terminal and lateral buds
• leaf scars and bud scale scars

Internal structure: the three tissue systems

Dermal tissue system: epidermis (which secretes a waxy cuticle)

Ground tissue: parenchyma cells

Vascular tissue system: vascular bundles composed of xylem and phloem

• in monocots they are scattered throughout the ground tissue
• in dicots they are arranged in a ring

in dicots the ground tissue is differentiated into cortex and pith

Growth in stems

Primary growth

Cell division occurs at an apical meristem

Cells lengthen by expanding vacuoles

Cells become specialized into different cell types

Secondary growth -- occurs at lateral meristems in woody plants (mostly dicots)

Vascular cambium -- between the xylem and phloem

Makes new (secondary) xylem on the inside, and new phloem on the outside

Old phloem constantly dies and sloughs off as bark

Cork cambium -- outside the phloem

Some trees have an extra cambium, creating the outer bark or periderm

Cork cells are dead and impregnated with suberin

Transport via xylem

stucture of xylem

• vessel elements
• tracheids
• sclerenchyma fibers
• xylem parenchyma

the tension-cohesion model

• water is lost from leaves through transpiration
• this generates tension in the spongy mesophyll...
• ...which pull water up from the xylem, which is possible because of water's property of cohesion

Note: xylem transport is FREE: it's powered by transpiration

Transport via phloem

stucture of phloem

• sieve tube members
• companion cells
• sclerenchyma fibers
• phloem parenchyma

the pressure-flow hypothesis

• at the sugar source:
  • sucrose is loaded into phloem cells at the source by active transport.
  • this causes water to be drawn in by osmosis from surrounding tissues, increasing pressure in the phloem sieve tube
• at the sugar sink:
  • sucrose is being actively transported out the phloem (for construction of new tissure, respiration or storage.)
  • this causes water to be drawn by osmosis out of the phloem, into the surrounding tissues, decreasing pressure
• the difference in pressures at source and sink results in the flow of water and sucrose through the phloem

Note: phloem transport costs ATP: it relies on active transport

Vocabulary

click here to go to stem vocabulary