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Primary Growth of Shoots
- The apical meristem of a shoot is domed shaped mass of dividing cells at the tip of the
terminal bud.
- As in the root, the apical meristem of the shoot tip gives rise to the Protoderm,
Procambium, and the ground meristem.
- Leaves arise as leaf primordial on the flanks of the apical meristem.
-Axillary buds have the potential to form branches of the shoot system at some later time.
- Lateral roots originate from deep within a main root.
- Braches of the shoot system originate from axillary buds, located at the surface of a
main shoot.
Primary Tissues of Stems
Vascular bundles- arrangements of vascular tissue to forma vascular bundle in the center
of the root.
- At the transition zone where the stem grades into the root, the vascular bundles of the
stem converge as the root’s vascular cylinder.
- Each vascular bundle of the stem is surrounded by ground tissue.
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Tissue Organization of Leaves
-The leaf is cloaked by its epidermis, with cells tightly
interlocked like pieces of a puzzle
- The epidermis provides defense against damage and the cuticle helps water loss.
Stomata – tiny pores flanked by special epidermal cells (guard cells) located in the
epidermal barriers. The stoma allows gas exchange between the surrounding air and the
photosynthetic cells inside the leaf.
Guard Cells - flank the stoma
Transpiration – the process of water loss from the plant via evaporation
Mesophyll – the middle eat of the leaf, between the lower and upper epidermis. It
consists mainly of parenchyma cells equipped with chloroplasts and specialized for
photosynthesis.
- The upper half of the leaf is the palisade parenchyma, made up of column shaped cells
- Below that region is the spongy parenchymas, which have air spaces for Co2 and O2 to
circulate.
- In most plants stomata are more numerous on the bottom surface of the leaf than on the
top. This helps to minimize water loss.
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Modular Shoot Construction and Phase Changes During
Development
Phase Changes- a gradual transition in vegetative growth from
a juvenile state to a mature state
Secondary growth: Lateral meristems add girth by
producing secondary vascular tissue and periderm
Secondary plant body- consists of the tissues produced during
the secondary growth in diameter.
- Two lateral meristems function in secondary growth:
Vascular Cambium – produced secondary xylem (Wood) and phloem
Cork Cambium – produces a though, thick covering for stems and roots that replaces the
epidermis.
-Secondary growth occurs in all gymnosperms.
- In angiosperms, secondary growth takes place in most dicots, but is rare in monocots.
Secondary Growth of Stems
Vascular Cambium and the Production of Secondary Vascular Tissue
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- The vascular cambium is a cylinder of meristematic cells that form secondary vascular
tissue
http://www.bio.txstate.edu/~dlemke/botany/1410lab/lab_exercises/lab5/sec_growth/sec_
growth.html
All of the cells and tissues that are found in the body of an herbaceous plant are produced
by apical meristems located at the growing tips of the stems and roots; such tissues are
called primary tissues. In woody plants, most of the tissues of the plant body are
produced not by apical meristems, but by specialized regions of cell division and growth
located elsewhere and known as lateral meristems. Tissues produced by lateral meristems
are termed secondary tissues, and the increase in plant size that they cause is termed
secondary growth.
Cortex. A well-developed cortex composed of parenchyma cells can usually be seen in
young woody stems.
Vascular Cambium. The lateral meristem responsible for producing the secondary
vascular tissues is known as the vascular cambium. Cell divisions in this meristematic
region produce the cells that differentiate into secondary xylem and secondary phloem.
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Secondary Xylem. The cells produced by the vascular cambium towards the inner side of
the stem differentiate into xylem. Since these cells are produced by a lateral meristem, the
tissue is referred to as secondary xylem. Like the primary xylem of an herbaceous plant,
secondary xylem is composed of parenchyma cells, sclerenchyma fibers, tracheids, and
vessel elements. Secondary xylem is what we commonly refer to as the wood of a plant.
Annual Rings. Each year, the vascular cambium produces new layers of secondary xylem
and secondary phloem. These layers are more conspicuous in the secondary xylem, where
they are recognizable as the annual rings in the wood. The age of a woody plant can be
easily determined by counting the annual rings in a cross section of the stem.
Secondary Phloem. The cells produced toward the outside of the vascular cambium
differentiate into secondary phloem tissue composed of parenchyma cells, sclerenchyma
fibers, sieve tube members, and companion cells.
Bark. The epidermis forms the protective outer layer of herbaceous stems and of very
young woody stems. Eventually, however, in woody stems a second lateral meristem,
known as the cork cambium, develops in the cortex. Cork cells, produced by this
cambium, combine wi th old, dead, secondary phloem cells to form the bark of the plant.
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