I. Secondary growth
A. In monocots and most dicots growth ceases with the maturation of the
primary tissues
B. In gymnosperms and woody dicots the roots and stems continue to increase
in diameter
C. This is called secondary growth and it results from the activity
of two lateral meristems:
1. Vascular cambium - gives rise to secondary xylem to the inside
and secondary phloem to the outside
a. Wood = secondary xylem
2. Cork cambium - gives rise to the periderm which replaces
the epidermis in woody plants
II. Vascular cambium
A. The meristematic cells of the vascular cambium are highly
vacuolated and exist in two forms
1. Fusiform initials - vertically elongated
2. Ray initials - horizontally elongated or squarish
B. Meristematic cells may divide in one of two ways:
1. Periclinal divisions result in the formation of two cells, one
in front or behind the other
a. Secondary xylem and phloem are produced through periclinal divisions
of the vascular cambium and their derivatives, i.e. the cell plate that
forms between the dividing initials is parallel to the surface of
the root or stem
b. Those divided off towards the inside become xylem
cells and those to the outside become phloem cells
2. Anticlinal divisions result in the formation of two cells, side
by side.
a. As the vascular cambium adds cells to the secondary xylem and the
core of the xylem increases in width the cambium is displaced outward. The
vascular cambium undergoes an increase in circumference via anticlinal
divisions of the initials
C. Technically vascular cambium refers only to the cambial initials,
however it is difficult to distinguish between these initials and their
derivatives so we often refer to a cambial zone
III.
A. In stems that undergo secondary growth the procambium between the
primary xylem and primary phloem retains the ability to divide. It will give
rise to the fascicular cambium, i.e. the cambium arising from within
the vascular bundles
B. In order to "round out" the stem parenchyma cells
between the vascular bundles form an interfascicular cambium
C. Eventually the fascicular cambium and interfascicular cambium join to
form a continuous ring around the stem
D. Much more secondary xylem than secondary phloem is added
1. The bulk of a tree is secondary xylem (wood)
E. As the vascular cambium moves outward the primary phloem is pushed
outward and crushed. Often all that remains visible are the primary phloem
fibers
F. With the initiation of secondary growth a periderm will form and
it will replace the epidermis
1. The periderm usually originates in a layer of cortical cells just
below the epidermis. The periderm is composed of three tissues:
a. Cork cambium - it is the middle, meristematic layer which gives
rise to cork to the outside and phelloderm to the inside
b. Cork (phellem) - formed to outside of cork cambium. Cork
cells are dead at maturity and the inner walls are lined
with suberin and wax, which makes impermeable to water. Quercus suber
is the source of cork bottle stoppers.
c. Phelloderm - it is a layer of living cells
just inside cork cambium. The cells resemble cortical parenchyma cells
IV. Annual growth
A. At the end of the first years growth a woody stem would have the
following (starting from the pith and moving outward): pith, primary xylem,
one ring of secondary xylem, vascular cambium, secondary phloem, primary
phloem, cortex, periderm (with three layers), epidermis
B. Each successive year more secondary xylem and phloem will be added to
the stem
C. The first formed periderm may last for several years
1. In older stems additional periderms may originate deeper in the bark
from parenchyma cells of the phloem no longer active. The manner in which
new periderms arise determines the external appearance of the tree bark
2. Bark = all layers external to the vascular
cambium. Therefore it includes secondary and primary phloem as well as the
periderm (and epidermis if still present)
D. Since living cells need to exchange gases, lenticels form where
portions of the cork cambium are more active, resulting in the formation of
tissue with many intercellular air spaces
E. Since the phloem is constantly being pushed outward and crushed, only
the innermost layers adjacent to the vascular cambium are functional phloem
F. As the stem thickens the inner, older non-conducting xylem is filled
with a variety of substances (oils, gums, resins, tannins) giving it a much
darker appearance. This is called heartwood. The outer, functional
xylem is called sapwood
G. The terms hardwood and softwood are
used to describe the woods of angiosperms and gymnosperms, respectively. They
do not refer to the actual hardness of the wood itself, e.g. balsa wood is
very soft but it is from a dicot
H. Stem development in a woody angiosperm:
1. Early
2. Late
3. Summary
