Natural Notes: Tree Biology

This post consists of my notes from a lecture on tree biology as part of a training to become a certified arborist by the International Society of Arborists.  The speaker was Sarah Rose, a naturalist with the Forest Preserves of Cook County and her lecture took place on January 14, 2015 at the Salt Creek Resource Management facility in Willow Springs, Illinois.  

Introduction
A tree is a long-lived woody perennial plant that can compartmentalize.  It is usually singe-stemmed, but not always.

Most plants are photoautotrophic (versus people who are heterotrophs).

Trees posses an apical meristem (discussed in detail below).  The cell walls of plants are composed of cellulose.  Trees are multicellular organisms that have a symoplasm (living tissue) and an apoplasm (non-living tissue).

Some trees are deciduous while others are evergreen.  In evergreen plants, only a portion of leaves all off at one time.  In deciduous trees, leaves fall off annually.


Roots
Roots are structures for water and mineral absorption.  Absorbing roots have root hairs that allows for water absorption (see Figure 1).  Absorbing roots are found within 18 inches of the soil.

Figure 1:  Diagram of absorbing roots.  

Root hairs are delicate and it is therefore very important to protect when handling plants.  The tap root anchors mainly young trees and stores sugar.  Water enters the roots through osmosis.  Water moves from the most concentrated area to the least concentrated area (see Figure 2).

Figure 2:  Osmosis of water in plants with absorbing roots.

Buttresses are roots that support the trunk (Figure 3).  

Figure 3:  Buttresses 

Adventitious roots do not form a primary root and may grow from stems (see Figure 4).  

Figure 4:  Poisson Ivy (Toxicodendron radicans) forms adventitious roots.

Most tree roots have a symbiotic relationship with mycorrhizae fungi (see Figure 5).  The plant provides sugars while the fungi have been known to assist the tree with nutrient absorption and their presence are important for healthy soil.  

Figure 5:  Mycorrhizae

Stem Anatomy

Figure 6:  Stem anatomy

The node is the location of a stem where there is growth (.e.g buds, meristem, etc.).  See Figure 7.  

Figure 8:  Location of girdle scar

Figure 7:  A node

A girdle scar is an old scar from former terminal buds and it goes all the way around the stem (see Figure 8).  Trees and many other plants exhibit apical dominance where the apical bud inhibits and controls growth of lateral buds.  Tree growth that has strong apical dominance is displays growth in an excurrent form (Figure 9A) while trees that have more lateral growth show deccurent form (Figure 9B).  

Figure 9:  Excurrent and decurrent growth on trees.  

Leaves

Leaves are often referred to by botanists as "food factories".  Indeed, they are the primary site of photosynthesis via their chlorplasts and stomata.  

Figure 10:  Parts of a typical leaf.  

Figure 11:  Leaf cross-section (upper area)

Branch Anatomy

Tree branches have a branch bark ridge where trunk tissues overlap with the branch tissues, as well as a bulging area where the trunk meets the branch bark known as the branch collar (see Figure 12).  

Figure 12:  Location of branch bark ridge and bark collar.  

The meristem is rapidly dividing, undifferentiated tissue.  It differentiates as needed.  It is found in the apical meristem at the terminal bud (Figure 6) and just behind the root cap (Figure 1).  Laterial meristems are found at nodes and within the vascular cambium, cork cambium, and the roots. 

 Primary growth in trees is from the primary (apical) meristem.  Secondary growth is from lateral meristems (vascular cambium and cork cambium).  

Vascular tissue

Vascular tissue moves products from source (e,g. the ground) to sink (e.g. a location on the tree) in a longitudinal direction. Xylem is the water and mineral conducting tissue.  Phloem transports photosynthate (sugars).  

Phloem is made entirely of living cells.  In gymnosperms, these cells are referred to as sieve cells.  In angiosperms, they are known as sieve tube elements and companion cells.  

Xylem provides shape to trees and stores some carbohydrates.  It also protects the tree from disease.  Xylem is made of a combination of dead and living cells.  The sapwood is the active portion of the xylem while the heartwood is less active and is made of mostly dead cells.  

Gymnosperms are composed of tracheids, fibers, and parenchyma cells.  Angiosperms are made up of tracheids, fibers, paranchyma, and also vessels.  These vessels are found within the vascular cambium.  

Wood

Figure 13:  Wood

A ray extends radially across the vascular tissue and serves as an alternate route for transport of water and nutrients if the tree is damaged (see Figures 13 and 14).    This is helpful as trees can undergo CODIT, or Compartmentalization of Decay in Trees.

Figure 14:  Rays.

Tree trunks can be ring porous, as in oaks (Figure 15) or diffuse porous, as in maples (Figure 16).  

Figure 15:  Ring porous wood

Figure 16:  Diffuse porous wood

Reaction wood forms in place of normal wood as a response to gravity (see Figure 17).  It is typically found in leaning or curving wood and is very strong. 

  

Figure 17:  Reaction (tension) wood

Photosynthesis

Carbon Dioxide + Water + Light = Conversion to glucose and oxygen

Photosynthate is used in conjunction with essential elements to make starch, amino acids, proteins, hormones, and pigments.  Photosynthesis occurs during the day when conditions are appropriate.  If there is not enough light or water, photosythesis is reduced.  

Tree Respiration

In respiration, oxygen and sugar are converted into carbon dioxide and water.  During dormancy, respiration slows but still continues.  

Transpiration

Transpiration occurs via the stomata.  Water moves through the plant to regulate temperature.  Water vapor exists through stomata underneath leaves (see Figure 18).  The opening and closing of the stomata is controlled by guard cells.  

Figure 18:  Stomata

Plant hormones

Auxin regulates growth and development and is responsible for apical dominance.  

Cytokinin is involved in cell division.  

Gibberellin is involed in cell elongation associated with phototropism and seed germination.  It is activated by water, temperature, and light.  Geotropism occurs in response to gravity (Figure 19).  

Figure 19:  Geotropism

Pigments

Chlorophyll (green)

Anthocyanin (red-purple-blue)

Carotenoids (yellow-orange-red)

Chloroplasts are within the chlorophyll and give it a green color.  

Monocots vs. Dicots

Monocots in their embryonic stage have a monocotyledon (Figure 20A) while dicots have dicotyledons (Figure 20 B).  

Figure 20:  Monocots vs. dicots

Monocots have scattered vascular bundles that do not form a ring as in the dicots (Figure 21).  

Figure 21:  Arrangement of vascular bundles in a monocot (left) versus a dicot (right).