Lecture
Notes
BIO 131, General
Botany
Jan. 22,
2009
We went over the first part of this on Thurs. Jan. 22. We will review
it on Tues. Jan. 27.
SECONDARY
GROWTH
As far as we're
concerned in this course,
SECONDARY GROWTH OCCURS ONLY IN DICOTS AND SOME GYMNOSPERMS (such
as pines).
SECONDARY GROWTH DOES NOT OCCUR IN
MONOCOTS.
Primary growth comes only
from apical meristems.
- Apical meristems are found at the tips of roots and stems (and in buds)
- Primary growth involves only growth in length of plant parts, not growth in diameter
- Differentiation of the
3 promeristems produces all the primary tissues and their various cell
types
- Primary growth always
is completed before secondary growth begins
Secondary growth comes
only from lateral meristems.
- Secondary
growth from lateral meristems does not occur
in monocots or in leaves. All tissues in monocots and leaves
are considered to be primary tissues
- Lateral meristems occur
in the form of cylinders which are parallel to the circumferences of
trunks, branches and roots
- Secondary growth
involves only growth in diameter of plant parts, not growth in
length
- The 2 lateral
meristems are the (1) vascular cambium (including its vascular
& interfascicular regions) and the (2) cork
cambium (= phellogen)
- The stem vascular
cambium is formed by residual procambium in the
vascular bundles, and by parenchyma cells in the pith rays which
dedifferentiate into meristematic cells and begin dividing. Once the
cambium has formed, it perpetuates itself.
- see Figure 26-7, p. 584 in textbook for a closeup
of a young vascular cambium.
- The root vascular
cambium is formed by the residual
procambium (in the notches between the xylem 'arms') and the pericycle
The vascular cambium
produces:
- secondary xylem
to the inside
- secondary phloem
to the outside
- parenchyma to extend existing rays or form new rays
In the picture above, the line ABOVE
where the red tissue (phloem fibers) stops is the vascular
cambium.
All tissue above that point is secondary xylem (=
wood).
Note the alternating
bands of fibers (bast) and sieve+companion cells in the phloem, plus the dilated
rays.
The periderm is
on the bottom.
The first cork cambium is formed by
dedifferentiated parenchyma in the outer part of the cortex.
This first cork cambium will usually last only a few years.
Early cork cambium that has arisen in
the outer cortex.
----------------------------------------------------------------------------------
The topmost layer of cells is the
epidermis.
Just
under the epidermis are some crushed-looking
cork cells. (cork =
phellem)
Below the cork cells is a layer of larger cells -
the cork cambium. (cork cambium = phellogen)
Below
the cork cambium are cells lined up in rows = the phelloderm.
The cork cambium
produces:
- cork (= phellem)
to the outside
- phelloderm to
the inside
The
cork layer produced by the cork cambium normally contains many more layers of
cells
than
the phelloderm (in other words, much more cork than phelloderm is
produced).
Lenticels are formed when very local areas of
the cork cambium "go wild" and produce abundant
cork cells. Lenticels allow the exchange of gases (mainly CO2 and
O2) between the living cells in stems and roots
and the atmosphere. There's a nice lenticel
picture on p. 587 in your textbook.
The
As the diameters of stems and roots
expand from secondary growth, several things happen:
The addition of
secondary xylem is mainly what causes stems and roots to grow in
diameter.
As the
vascular cambium forms secondary xylem, the interior of the stem or root gets
larger. Because of the added volume of xylem in the inner stem or
root, tissues outside of the vascular cambium (primary phloem and older
secondary phloem) soon are squeezed, the sieve cells and companion cells are
more or less crushed and become nonfunctional. That's the reason why the
functional phloem in a woody stem or root is found only very
close to the cambium. Sieve cells actually are functional only for
about one growing season.
Note the triangular group of parenchyma
cells above. They make up a phloem ray.
The ray has greatly expanded to help fill the
expansion of the stem caused by secondary
xylem.
As the circumference of
the stem continues to expand, the original cork cambium stops functioning, and a
new cork cambium (or several of
them) is formed by the
dedifferentiation of parenchyma cells in the inner cortex and the phloem rays (in roots, the pericycle also may help form this second-stage cork
cambium). This cork cambium functions for a few years
and then it also is pushed too far out by the added secondary xylem, so it stops
functioning.
Be sure to study carefully the following: The process of becoming woody, illustrated by Figures 26-7, 26-8, and 26-9, on pp. 584-585 of the textbook, PLUS how a lenticel is formed (Figures 26-10 and 26-11, on pp. 586-587 of the textbook). Also, study the pictures on p. 588 of the textbook.
Be able to define the following terms, what they are composed of, and know what their function(s) are: sapwood, heartwood, softwood, hardwood, bark, wood, early or spring wood, late or summer wood, annual ring, tyloses, bast fibers.
ORIGINS OF SECONDARY (= LATERAL)
MERISTEMS
Summary outline
MONOCOTS
no secondary meristems in stems or roots
DICOT (and pine)
STEMS
1 Vascular cambium +
interfascicular cambium (both of these together are just
called 'the
cambium')
originate from residual procambium and dedifferentiated pith ray
parenchyma
2 Cork cambium (=
phellogen)
First cork cambium forms from dedifferentiated parenchyma
in outer part of the
cortex
Later cork cambiums form in secondary phloem (and
phloem ray cells)
DICOT (and pine)
ROOTS
1 Vascular
cambium (technically, there is no interfascicular cambium,
because roots have no pith
rays)
2 Cork
cambium
First cork cambium forms from
pericycle
(yep, that's correct, but look in vain for it in books and on the web)
Later cork cambiums form in secondary phloem
end of notes for Thursday Jan. 22