Support systems II- Trees LECTURE 4

How do plant cells cupport themselves (unlike animal cells?)

• internal hydrostatic pressure

• acting against the cell wall

How?

• impressive hydrostatic pressures of more than 10 atmospheres

  → without bursting

  → consequence of Laplace’s law

Why no locomotion?

• inflexible walls

• cells of plants and fungi are not suitable for forming muscles

• require large and rapid cell deformations

Two main reasons for tree stability problem

1. own weight (gravity forces)

2. wind (drag forces)

Which is most important problem?

Gravity forces?

• long thin columns can buckle and collapse but never really happens

• trees are much thicker

• In side branches?→ no→ CSA is still circular

Wind forces→ more important

• can also uproot or break trees!

Bending moment on tree

M= Fwind x h

Resulting stress on outside of trunk

stress= 4M/(r³pi)

The bending moment sets up stresses in the tree trunk

• highest on the outside

When do stresses increases/decrease

• stress increases linearly with tree heigh and leaf sail area

• decreaess with the cube of the stem’s radius

To reduce excessive stressess the tree could

1. grow less tall

2. grow thicker trunks

3. reduce the leaf area

Instead of removing photosynthetically active leaves, many trees…

• have leaves or entire stems that can strongly deform (reconfigure)

• in the wind

• giving them a streamlined shape that reduces drag

Structure of wood

• lightweight cellular structure

• mainly xylem conduits

• only cells at exterior part of the stem are alive

Material properties are direction-dependent

→ wood is anisotropic

90-95% of all the cells are aligned parallel to stem

• remainder are storage cells that are arranged in the radial direction

• no cells are aligned tangientially

Conifer vs Angiosperm wood

Conifer wood (softwood)

• 1-10mm long and thin ttracheids

• 10-80microm diameter

Angiosperm wood (hardwood)

• thin fibres and very long 10cm-10m

• wide vessels (diamter 30-300 microm)

• several joined cells

What are wood cell walls made of?

• microfibrils of cellulose

• embedded in matrix of hemicelluloses

• lignin

→ several layers

The several layers of wood

• middle layer→ thickest

• orientation of cellolose microfibrils in this layer→ important for te wood’s mechanical properties

Strucuture of the fibrils

• oblique

• wound helically

• at an angle of ca 5 - 50 degrees to the tree trunk axis

→ The larger the angle, the softer and the more extensible is the wood

Fibril angle in pine trees

• decreases from the pith to the bark

• reflecting differences between young and old trees

Why have this?

• reflect a change in strategy from young to old trees

• younger trees are relatively more exposed to the wind due to their larger surface area

  → escape the wind forces by aligning with them

  HOW?

  • they are more flexible

• Older trees

  • heavier and cannot afford bending over too much

    → designed to be stiffer and to resist the bending

Compression vs tension in wood

• wood cells are thin-walled tubes

  → prone to buckling

THERFORE: only about half as strong in compression as in tension

How do trees reduce compressive stresses

Pre-stressed

1. fully hydrated wood cells are laid down by cambium on outside of the stem

2. dry out and shorten as they mature

3. as they are attached to wood inside, this sets up ‘pre-tension’ in the outer part of the stem and ‘precompression’ in the middle

4. This prestress has the advantage that the max compressive stresses are reduced when the stem is bent over by the wind

How do conifers respond to load on branches?

• adding compression wood

  • on the underside with large microfibril angles

How do angiosperms respond?

• adding tensile wood

• with small fibril angles

• on the upper side which tends to shorten

Thigmomorphogenesis

• trees control mechanical design

• etecting mechanical stresses and responding to it with a range of growth responses→ thigmomorphogenesis

In plants prevented from bending (staked trees)

• stems and roots grow insufficiently

• so that the stem is unstable when the stake is removed

Where do trees lay down wood?

• where mechnical stresses are highest

• indicating growth responses of the wood are controlled locally

• deposition of new wood by secondary growth in cambrium reduces the mechnical stress

  • provides a feedback system that keeps stresses low

Experiements have shown (what is needed to stimulate growth responses?

• only a small number of short pertubations (stresses)

How are mechanical stresses detected in plants?

1. via opening of stretch-sensitive ion channels

2. trigger further intracellular reactions

3. effects of mechanical pertubations are axuin-stimulated growth and callus deposition

How do trees ensure they are anchored in the soil?

1. Lateral roots

   • withstand torque of the wind

   • wider base shifts, the centre of rotation sideways

   • giving the soil weight more impact because of the larger lever arm

     → often laterally flattened and thus well-adapted for vertical forces

2. Compressive buttresses

   • have lateral roots but also grow a massive base→ helps them to resist compressive loads

3. Tensile buttresses

   • some trees grow long and thin buttresses→ too weak to resist compression

   • but can develop tension when the roots are well anchored

4. Tap roots

• long vertical taproots that reist wind forces by puhsing soil sideways

Another strategy to help reach large heights?

• climb

  → climbers attach themselves to rocks or other plants using

• twining stems, hooks, clinging roots or tendrils→ adhesive pads

• Attachments are purely tensile→ stable with much less material