Roots

Goals

1. To understand how roots are put together

2. To become familiar with the composition of soil and its role in root function

3. To understand how roots function

Outline

Introduction

Functions of roots:

• anchoring & support
• absorption of water and nutrients
• storage of organic molecules

Root structure:

monocots vs. dicots

• dicots generally have a tap root system often with lateral roots
• monocots have a fibrous root system

general structure

• root cap
• zone of cell division: the apical meristem
• zone of cell elongation
• zone of cell maturation

tissue systems

• Dermal tissues systme: an epidermis that gives rise to root hairs in the zone of maturation
• Ground tissue: mainly present as cortex, the inner layer or which is the endodermis
• Vascular tissue: the stele or vascular cylinder in the center of the root
  • the outer layer is the pericycle, which can give rise to lateral roots
  • the xylem and phloem are found in the stele (xylem inner)
  • in monocots the stele has a pith

growth

• 1° growth from apical meristem, lateral roots from pericycle
• 2° growth in dicots from vacular cambium between xylem and phloem

Soil

Five elements of soil

• inorganic mineral particles
  • differences in particle size
  • differences in chemial composition of particles
• organic matter
  • humus
  • feces
  • corpses, etc
• air
• water
• organisms

What makes good soil good?

A range of soil particle sizes (as in loam) improves water infiltration, retention and cation exchange potential

Humus improves the ability of soil to hold water and nutrients, and decomposes to release nutrients

How are nutrients and water present in soil?

Water may be unavailable (bound to particles) or free

Cations tend to adsorb to soil particles

Anions tend to be free in soil water

How do plants get nutrients and water from soil?

Water and free nutrients (anions and cations when abundant) can be taken up by active transport/osmosis

Bound cations are taken up by means of cation exchange

Uptake by roots

Water and dissolved minerals must travel from outside the root to the stele, if they are to go to the rest of the plant

They may go via the symplastic route (through cells and the plasmodesmata that link them [i.e., the symplast]), or via the apoplastic route (through the apoplast -- between cells). Once they reach the endodermis, however, the Casparian strip forces them into the symplast.

Dissolved minerals are pumped in via active transport, and water will follow as internal water potential falls.

Mutualisms

Rhizobium & legumes -- Nitrogen fixation in nodules

 Mycorrhizae -- a mutualism between fungi and roots

Predation -- insectivorous plants

Vocabulary

click here to go to root vocabulary