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Structure of woody plants (Concept (primary growth (cortex, Vascular…
Structure of woody plants
Concept
primary growth
cortex
Vascular bundles
epidermis
pith
leaves
all in herbs
in a woody species additional tissues are produced in the stem, root from other meristems,
cork pro cambium
vascular pro cambium
the new tissues themselves are wood and bark
they come as secondary tissues
woody plants not only become taller through growth by their apical meristem but also become wider by accumulation of wood and bark
because wood and bark contain conducting tissues, their accumulation gives plants a greater capacity to move water and minerals upward and carbohydrates downward
the number of leaves and roots that the plant can support increases as does the photosynthesis capacity
even if a ring of wood could conduct for only 1 year, the plant could still produce a greater number of leaves every year
its annual photosynthesis capacity would always increase.
the consequence of this ever-increasing capacity is that annual production of seeds that germinate in a suitable site are able to grow into adults and reproduce
Secondary growth also has some disadvantageous
bigger target for pathogens
battles insects, fungi, and environmental harshness 10,000 times longer
very expensive to condruct bark and wood
greater need for defense both chemically and structurally
woody plants are a combination of primary and secondary tissues
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the tips of stem and roots as well as the leaves, flowers, and fruits are herbaceous and primary
only as portions of stems and roots become older do they begin to undergo secondary growth and become woody
Secondary Phloem
pic
is formed from the vascular cambium just as the secondary xylem
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it too has an axial and radial system
the axial ssyetm is responbile for conduction up and down the stem or root; ot contians sieve tube members and companion ells in angiosperms, or sieve cells in conifers
in both groups of plants, fibers and nonconducting parenchyma are also typically present in axial secondary phloem.
the size, shape and number of phloem rays match those of xylem rays because both are produced by the same ray initials.
phloem rays consist only of parenchyma cells that are used for storage as are xylem rays but phloem on conifers, albuminous cells ray cells
vascular Cambium
Initiation of the vascular cambium
is one of the meristems that produce the secondary plant body
cells located in this position never undergo cell cycle arrest,
they continue to divide instead of maturing and they constitute the
Fascicular Cambium
Fascicular Cambium
bundle
pic
some mature parenchyma cells between the vascular bundles come out of cell cycles arrest and resume mitosis forming an
interfascicular cambium
interfascicular cambium
pic
connects on each side with the
Fascicular Cambium
vascular cambia must be extended each year.
the tips of roots and stems initially contain only primary tissues; however at the some time after the metaxylem and metaphloem have matured
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a vascular cambium arises and that portion of the root or stem then contains both primary and secondary tissues
during the next growing seasonm the apical meristem extends the axis beyond this point
a new segment of V.C. formed in the previous season
The V.C. forms in leaves that stay on a tree for many years, but just a tiny amount of secondary tissues, usaually only secondary phloem woth out any seconday xylem is formed in the mid rib
the other vein contain only primary tissues.
Fusiform Initials
pic
are long tapered cells in conifers
when they undergo longitudinal cell division with a parallel to the circumference of the cambium (Periclinal wall)
it produces two elongated cells
the other differentiates into a cell of secondary xylem or secondary phloem
#
if the outer daughter cell remains a cambium cell, the inner cell develops into secondary xylem
one continues to be fusiform intiial
Periclinal wall pic
wood never forms to the exterior of he V.C. and bark never form from the interior side
daughter cells are located on the inner side, which mature into secondary xylem increase greater diameter causing cambial cells to be pushed outward
anticlinal walls
perpendicular to the cambium's surface
pic
increaing the number of cambial cells
without it, Cambial cells would stretch wider circumferential and finally wont function
like apical meristems, F.I. have a thin primary cell walls and plastids are present in the proplastids
after nuclear division, a phragmoplast forms and elongates toward to ends of the cells
Ray initials
similar to Fusiforms but that they are short and more of less cuboidal
pic
they a too undergo periclinal cell division
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one difference that the elongated Fusiform produces cells of wood and phloem,
Rays produces short cells, mostly just storage parenchyma and in gymnosperms and albuminous
Arrangement of Cambial cells
fusiforms and rays are organized in a specific pattern
pic
Rays are typically grouped together in short vertical rows only one cell wide, two or many
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fusiform may occur in regular horizontal rows or irregular without any horizontal pattern
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stored cambia have evolved more recently than non storied and occur in only a few advanced eudicot species
the fusiforms of stored tend to be short
#
if a group of rays get to larger they will convert into fusiforms
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outer bark
Cork and the cork cambium
as the youngest most innermost phloem cells for and mature, they contribute to the larger diameter of the stem or root and increae pressures acting on the outermost tissues.
this requires that tissues on the periphery of the plant either grow in circumference or be torn apart
actually, the tissues do both but both must be controlled.
the intergrity of the plants surfaces must be maintained against invasion by fungi, bacteria and insects
pic
maturing cork cells increase slightly in volume, the their thin primary walls becomes encrusted with suberin making them water-probably a critical part of maturation because the protoplasm breaks down
outer bark
all tissues outside the innermost cork cambium comprise the ?
pic
inner bark
all secondary phloem between the vascular cambium and the innermost cork cambium is the ?
pic
Lenticels and Oxygens Diffusion
the impermeability of cork has negative as well advantageous consequences.
keeps out pathogens and retains water, it also blocks absorption of oxygen, interfering with the respiration of the sapwood vascular cambium and the inner bark
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bark becomes permeable to oxygen when cork cambium produces cork cells cannot fit toughly together, intercellular spaces penetrate the cork layer creating a diffusion pathway for oxygen
when a new cork cambium arises interior to this ones, it too forms a lenticel in the same place
the outer and inner lenticels are aligned, permitting oxygen to penetrate across all layers of the bark
Lenticel-producing regions that produce only ordinary impearable cork, consequently lenticels contain more layers of cells and protrude outward.
Initiation of cork cambia
timing of initiation of its first cork cambium is far more variable than that of the vascular cambium.
in some species the first cork cambium arises before a twig or root is even 1 year old. on stems this is often detectable as the surfae color changes from green to tan.
in other species the first cork cambium forms only seven years old, until then the epidermis and cortex are retained.
epidermises more than 40 years old have been reported.
delayed formation of bark is common in plants that depended on cortex chlorenchyma for much of their photosynthesis as cacti do
the first cork cambium may arise in a number of tissues, epidermis , cortex, primary phloem, or secondary phloem
Subsequent cork cambia may form shortly afterward, sometimes in the same season, but usually a year or two later.
if the growth in diameter is slow, new cork cambia may arise at intervals of as much as 10 years
these later cork cambia usually form deep in the secondary phloem
Secondary Xylem
Types of woods cells
all cells formed in the interior are secondary xylem known as wood
contains all types of cells that occur in primary xylem but now new ones
wood may contain tracheids, vessel elements, fiber ,sclerids and parenchyma
they only real difference between the primary and secondary xylem is the arrangement of cells
the arrangement of secondary xylem reflect that of fusiforms and ray
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Axial systems are from fusiforms
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always contains tracheary elememts, which carry out longitudinal conduction of water through woods
in some species, the axial also contains fibers that give the wood strength and flexibility
radial systems are form ray
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Hardwoods
wood of all basal angiosperms and eudicots, even those that lack fibers or are very soft
softwoods
woods from conifers have few or no fibers thus have soft consistency.
can be harder than many hardwoods
tracheary elements and fibers are elongate, as are fusiform that produce them
Ray tracheids
horizontal, rectangular cells that look somewhat look like parenchyma but have secondary walls circular bordered pits and protplasts that degenerate quickly after secondary wall is complete
pic
Growth Rings
pic
this regions with strongly seasonal climates, the V.C. is quiesecnt during times of stress
either winter cold or summer drought, but when the Q.C. the V.C. becomes active the cell division begins
at the same, the new expanding leaves are thin and delicate and their cuticle is neither thick nor fully polymerized.
leaves like this lose water at a rapid rate, thus trees need a high capacity for conduction at this time
early wood
also known as spring wood and it must have a high proportion of wide vessels
pic
or in conifers, wide tracheids later, the cuticle has thickened, transpiration is less , and large numbers of newly formed vessels are conducting rapidly
late wood
pic
or summer can have a low proportion of vessels
but the plant is a year older, is larger and heavier and it needs more mechanical strength to hold up the increased number of leaves and larger branches.
late wood is stronger if it contains numerous fibers or in conifers if it contains narrow thick-walled tracheids.
finally at the end of the growing season , the cambium becomes dormant again.
the last cell often develop on as heavy fibers with especially thick secondary walls
in a tree with wood that just described it is easy to see early wood and late wood, the two together making up 1 year's growth an
annual growth
pic
Heartwood and sapwood
pic
the center of a log is almost always darker in the color then the outer wood,
and it is usally drier and more fragant.
the dark wood is
heartwood
and the lighter moist region is the
sapwood
the different regions exist because the vessel and tracheids do not function forever in water conduction
water columns break because of freezing, wind vibrations , tension, wood-boring insects and other factors
after the water column breaks, there is no means of pulling water upward; vessels and tracheids in which this has occurred usually never conducted water again
only a few water columns break at any time, after several years all water columns in a growth ring have snapped and that ring no longer conducts
new water filled tracheary elements are produced by the cambium during the next year
an important problem is that a vessel is wide enough that a fungus can easily grow up through it
for the vessel elements that are not conducting, a mechanism that seals them off is selectively advantageous
wood parenchyma cells adjacent to vessels push bubbles of protoplasm through the pits into the vessel, forming a plug called
tylosis
pic
occurs repeatedly until vessel is filled
Ultimately all parenchyma cells die and the conversion of the sapwood to decay resistant heartwood is complete
a new layer of sapwood is formed each year by the vascular cambium and on average the one annual ring is converted to heartwood each year
reaction wood
pic
in branches or trunks that are not vertical, gravity causes a lateral stress, if not counteracted, the branch would droop and become pendant
reaction wood
in response to such stress most plants produce
in angiosperms, this develops mostly on the upper side of the branch and is known as tension wood
in a croos section of such branch, growth rings are eccentric, being much wider on the top of t he branch.
tension wood contains many special gelatinous fibers have walls are rich in cellulose but have little or no lignin
these fibers exert tension on the branch, preventing it from drooping or the tension wood may even contract , slowly lifting a branch to a more vertical orientation
