Secondary GrowthChapter 5

Secondary Growth

• Secondary growth is an increase in girth of a plant initiated by cell divisions in lateral meristems.

• In woody plants, primary growth occurs simultaneously with secondary growth.

Compare and Contrast 1° and 2 ° Growth

Secondary1. Growth in girth2. Lateral or Secondary

meristems (i.e. vascular cambium and cork cambium)

3. Cylindrical meristems4. Occurs in regions of a

woody plant where primary growth has ceased

Primary1. Growth in length2. Apical or Intercalary

meristems (i.e. protoderm, procambium, and ground meristem)

3. Meristems are clusters of undifferentiated cells

4. Occurs in regions of new growth of the plant

Lateral Meristems• Differentiated cells (from primary growth) revert to

become undifferentiated cells, forming the lateral meristems (Cambium). (Latin cambire, “to exchange”. Cambium cells have “exchanged” their previous roles for a new role of dividing repeatedly for new growth.)

• Vascular Cambium: produces secondary xylem (to the inside of the cylindrical meristem) and secondary phloem (to the outside) (called “secondary” xylem and phloem to distinguish it from the vascular tissue formed from the procambium during primary growth.)

• Cork Cambium: produces new dermal tissue; also known as phellogen (Greek phellos, “cork”, genos, “birth”)• If the cork cambium did not form, the loss of the ground tissue and

epidermis would leave the tree without a protective outer covering (water loss and infection would increase.)

Lateral Meristems:

Vascular Cambium and

Cork Cambium

Secondary Xylem• Expands the plant’s capacity to carry water and

minerals up from the roots• Adds structural support• New secondary xylem replaces old secondary xylem

that no longer conducts• Only recently formed layers of secondary xylem

conduct water and minerals with the older primary and secondary xylem tissue being inactive.

• Secondary xylem is the “wood” of a tree (Greek xylon, “wood”)

• Vascular cambium produces much more secondary xylem than secondary phloem

Lignin• Secondary Xylem is full

of lignin, a rigid molecule in the cell wall of secondary xylem, which adds considerable strength to the cellulose backbone of cell walls.

• Lignin constitutes up to 25% of dry weight of wood and is the second most common organic compound on Earth, cellulose being the first.

Secondary Phloem– Increases transport of food from the leaves to the

rest of the plant body– Only recently formed layers of secondary phloem

conduct phloem sap, with the older primary and secondary phloem tissue being inactive.

– Older phloem cells no longer conduct because they are stretched and broken when new cells produced by the vascular cambium push them outward.

– Eventually becomes part of the bark

Bark• Bark consists of all the tissues external to

the vascular cambium (everything outside the wood)

• Two distinct regions in bark– Inner bark: consists of living secondary phloem,

dead phloem cells, and any remaining cortex (ground tissue)

– Outer bark: consists of dead tissue, including dead secondary phloem and periderm

• (Periderm is protective tissue replacing the epidermis of young plants, includes the cork.)

BARK!Paper birch (Betula papyrifera)

Black oak (Quercus velutina)

Shagbark hickory (Carya ovata)

Sycamore, or buttonwood (Platanus occidentalis)

Heartwood: old layers of xylem that no longer transport water

Sapwood: consists of younger xylem that actually transports water -Point to the vascular cambium on this picture.

-Point to the cork cambium.

Growth rings (“Tree rings”)

• Growth rings are visible because of the distinction between early wood produced in spring and late wood produced in late summer or fall.

• Spring and early summer = large cells with relatively thin secondary cell walls

• Late summer and fall = smaller cells with thicker secondary cell walls

Annual rings

• Each annual ring in wood generally represents one year’s increment of growth

• The number of rings varies with the distance above ground.

Dendrochronology

• Dendrochronology is the science method of dating based on the analysis of patterns of tree-rings, often has implications with climatic interpretation.

• (From the Greek dendron, “tree”, and chronos, “time”)

• Tree rings reveal a tree’s age and details about climate and human history. How?

• What would 20 thin rings and 2 thick rings indicate?

Dendrochrology cross-dating• The analysis of tree ring patterns

to determine climate patterns, to date archaeological artifacts, etc… by matching ring-width patterns between samples from living trees and dead wood.

Dendrochrology cross-dating

http://www.youtube.com/watch?v=7v83X_3ENfU (First minute intro and 17:54-22:45)

Dendrochronology and Environmental Studies

• Mastering Science- Lord of the (Tree) Rings: What trees can teach us about environmental historyhttp://www.youtube.com/watch?v=fFcENDkXrZA

• Tree Rings: Counting the years of global warming.http://www.youtube.com/watch?v=vqicp4PvHrY

Some Commercial Uses of Wood and Bark

Paper made from a liquid solution of ground-up wood

Cork Cambium• When secondary growth begins,

epidermis sloughs off and is replaced with cork made by the cork cambium.

• Cork is harvested to make... CORK!

Latex from various plants, used to make various products including rubber (see p116 about rubber production)

Lectures built from the textbook, Introduction to Botany, by Murray W. Nabors.

Lumber!