leaf: growth leaf: external leaf: internal leaf: modified

Stem/ woody

Woody stems:

Using the diagram below as a guide, work through the slides images of woody sections from conifers and angiosperms.

Meristematic regions:

a. Periderm: the cork cambium divides toward the edge to form the cork and towards the center to produce phelloderm cells.

Cork is critical in that it keeps out both biological and environmental elements which might cause harm. In areas where fires prevail, cork may be many inches in thickness. It insulates against the heat, preventing denaturation of the tissues below.

In tropical areas, the bark ( periderm + phloem) may be shed periodically, This may help in preventing the buildup of plants ( including algae) that mat on the surface..

Heavy wax buildup on some barks repels water, which were it to moisten the surface would encourage fungal growth and prevent the leaching of cellular material.

See your text for patterns of bark growth...

b. The vascular cambium, which we have previously covered in stem growth, here is at its' prime. Divisions towards the center produce a complex tissue of xylem rays, vessels, tracheids, and parenchyma. Divisions in the directions of the edge produce a complex of phloem tissue: sieve cells, companion cells, fibers, parenchyma and phloem rays. Phloem tissue does not last long, as it caught between the outwardly extending xylem and the periderm. It is replaced annually or there about.

Pith and Cortex: both have short life spans in woody plants, generally disappearing within the first 4-5 years. As the xylem increases with its rigid ligninfied walls, parenchyma of the cortex and pith are crushed.

 

We can compare the 3 cuts of pine and oak: cross section, radial ( cut parallel to the rays, as
with cutting a slice of pie) and tangential ( cut at an angle or tangent to the stem):
look for ray positioning & tracheid dominance to identify what type of cut is involved......

pine radial oak radial
pine tangential: oak tangential

pine cross:

oak cross

 

Not all hardwoods are ring porous as the oak; others are diffuse porous as Tilia: below. In ring porous ( see the oak cross section ), the xylem vessels laid early in the season are large with thin walls to expedite the fow of water or sap. Later in the season, as moisture becomes more scarce, the cells may not be as wide, and more secondary walls are laid leading to a smaller open volume in the center but a stronger wall structure. In diffuse porous, the cells remain the same size throughout the season.

Last but not least, although monocots do not form secondary wood tissue, they can become large i diameter...explain what you see below. Use your text to explain the phenomenon!

ß