Chapter 31, pages 668-691
Plant Anatomy-2 0:00-10:29
Plant Anatomy-2 14:44-33:32
I. Introduction
A. Primary growth is responsible for:
1. Growth in length of the stem
2. Produces the basic tissue pattern in the stem
B. Secondary growth is responsible for:
1. Growth in girth of the stem
2. Produces secondary (new and more) vascular tissues
3. Provides a continuous
connection of
meristem
cells between the primary tissues of
the roots
and the primary tissues of the shoots.
4. This is why some trees
(California Redwoods and Bristlecone Pines) to live to such a
great age
(several thousand years)
II. Development of the Vascular Cambium
A. The function of the vascular cambium is to
produce
secondary
growth, thus the vascular
cambium must be formed before
secondary growth can occur
B. Two regions of the primary stem contribute to the vascular cambium
1. Fasciscular cambium - the meristem cells within the vascular bundle
2. Interfascicular cambium - the meristem cells between the vascular bundles
C. The differentiation of the fascicular cambium
1. Not all of the procambium in the vascular bundle differentiates into xylem or phloem
2. This undifferentiated procambium is called the residual procambium
3. The residual procambium is 2-4 cells wide and remains meristematic
4. The residual procambium
will be called vascular cambium when the
vascular
cambium begins to divide to form secondary tissue
5. Thus, fascicular
cambium is the regions of the vascular cambium that originated
within
the vascular bundle
D. The differentiation of the interfascicular cambium
1. This portion of
the vascular cambium originates in the
pith
rays between the vascular
bundles
2. A band of
parenchyma
cells, about 2-4 cells wide de-differentiate and become
meristematic
3. When these cells
begin to divide they are called vascular cambium and are referred
to
as interfascicular cambium because of where they originated
E. The vascular cambium
1. Becomes vascular cambium
when the fascicular and interfascicular cambium join to
form a complete
cylinder around the stem
2. As soon as the
cylinder
is formed the vascular cambium becomes active by dividing
on both the
inner and outer surface of the vascular cambium surfaces.
3. Activity of the vascular cambium
a.
New xylem cells are formed inwardly and attached to the previously produced
xylem
b.
New phloem cells are formed outwardly and are attached to the previously
produced phloem
c. Two kinds of vascular cambium cells exist:
(1) Ray initials
i. Produces the radial (lateral) transport system cells
ii. In xylem and phloem these are the parenchyma cells
i. Produces the axial (vertical) transport system cells
ii. In xylem and phloem these are
the
sieve-tube
member,
companion
cells,
tracheids,
vessels,
and
fibers
d.
Generally the xylem-producing cells are more active than the phloem-producing
cells
e. The cambium is active from the spring to the fall and is inactive in the winter
f. The yearly activity of the cambium produces the annual rings in the xylem
II. Periderm (cork)
A. Function of the periderm:
1. Increase in diameter of the stem occurs with the activity of the vascular cambium
2. This causes the protective epidermis to crack and split open
3. Thus, a need for
a meristematic layer at the outer edge of the phloem for the internal
protection
of the stem
4. Thus a layer of cork cambium forms outside of the phloem.
5. The cork cambium forms a layer of waxy cork cells
6. The cylinder of cork cambium increase in diameter as the stem increases in diameter
B. Formation of the cork
1. In the young stem (1 year old or less)
a. Cortical cells just under the epidermis become meristematic
b. Produces a layer 1-2 cells thick of cork cambium called phellogen
c.
Phellogen produces a layer of cork cells 4-6 cells thick external (toward
the
epidermis) to the phellogen
d.
Phellogen produces a single layer of cells,
phelloderm,
internal (toward the
xylem and phloem) to the phellogen
2. Structure of the cork cells
a. Cells are flattened
b. Cell walls contain suberin, a waxy substance
3. In old stems (more than 1 year old, generally 3-4 years)
a. A new phellogen forms because the former phellogen dies
b. The new phellogen forms in the outer region of the still-living phloem
c.
New phellogens will form about every one to four years depending upon the
species of tree
C. Bark
1. As the layers of
cells outside the vascular cambium die, they are sloughed off as
bark
2. In the young stem
the bark contains:
epidermis,
cork,
cork
cambium,
phelloderm,
cortex,
and
phloem
3. In the old stem the bark contains: cork, cork cambium, phelloderm, and phloem
III. Other Points about Woody Dicot Stems
A. Microscopic Sectioning
2. Radial Section - longitudinal section on the diameter of the stem
3.
Tangential
Section - longitudinal section on the tangent (perpendicular to the
radius)
of the stem
B. Wood - xylem tissue
C. Softwood - wood with only tracheids in it
D. Hardwood - wood with both tracheids and vessels in it
E. Heartwood - wood in the center of the tree, no longer conducting
F. Sapwood - wood at the periphery of the stem, actively conducting
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