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Structure of Woody Plants, secondary xylem is wood, secondary phloem…
Structure of Woody Plants
Concepts
in herbs, Primary Tissues (from apical meristem) like epidermis, cortex, vascular bundles, pith and leaves form the entire plant
Secondary Tissues form in woody species
these become wood (secondary xylem) and bark (secondary phloem and cork)
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these form in the vascular cambium and the cork cambium
trees such as chestnuts, sycamores and shrubs roses and azaleas
woody plants become taller by growth at apical meristems but also grow wider by accumulating wood and bark
wood and bark contain conducting tissues, meaning it can support more leaves and therefore more photosynthesis
pines oaks and others can live 600 years, others even longer from 1k to 5k yrs
wood and bark are relatively expensive metabolic products
Woody plants came before herbs, early angiosperms were all woody.
Vascular Cambium
Initiation of Vascular Cambium
Vascular Cambium is one of the meristems that produce secondary growth
in herby plants these cells stop dividing and differentiate into conducting tissues
in woody plants they continue dividing and are called the fascicular cambium
some mature parenchyma cells in between vascular bundles come out of cell cycle and resume mitosis forming Interfascicular cambium
vascular cambium is a rather simple meristem. it forms only two types of cells, fusiform initials and ray initials.
Fusiform Initials
fusiform initials are long tapered cells, typically 140 to 562 micrometers in dicors and 700 to 8200 micrometers in conifers
when a fusiform initial undergoes division with a wall parallel to the circumference of cambium (periclinal wall) it makes 2 elongate cells
one stays fusiform initial and the other daughter becomes secondary xylem or phloem
if the outer cell remains cambium the inner becomes secondary xylem
if the inner remains cambium the outer cell matures into secondary phloem
wood never forms to the exterior of the vascular cambium and bark never forms on the interior side
xylem is wood, phloem cortex and epidermis make up cork cambium or bark
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Ray Initials are similar in initiation but form short cuboidal cells for storage parenchyma and albuminous cells
Fusiform Initials can occur in regular horizontal row Storied Cambium or irregular Nonstoried Cambium
A single tree or shrub has only 1 single vascular cambium one cell thick extending upward in the trunk!
Secondary Xylem
Types of Wood Cells
all cells formed to the interior of the vascular cambium develop into secondary xylem, known as WOOD
wood may contain tracheids, vessel elements, fibers, sclereids and parenchyma
The arrangement of secondary xylem is in fusiform and ray intials
axial system is developed from the fusiform initials
radial system develops from ray initials
commercially important woods usually come from trees with lots of fibers or hardwoods
pines and redwoods contain few or no fibers and are known as softwoods
secondary xylem is wood
secondary phloem becomes part of bark
Secondary XYLEM - WOOD
