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Structure of Woody plants (concepts (consequences of secondary growth…
Structure of Woody plants
concepts
herbs can produce adventitious roots to survive winter :snowflake:
woody plants growth in height due to apical meristem and in width due to accumulation of cork/bark/wood :straight_ruler:
consequences of secondary growth include
limit of conducting capabilities :red_flag:
all provascular cells differentiate into xylem or phloem :lock:
helpful in water retention however due to limiting of leaves :potable_water:
seeds like to germinate in favorable spots/microhabitats, if none are found seed will not grow
secondary tissues include wood, bark and cork found in trees and shrubs
more disadvantages of secondary growth include battle of environmental elements and insects for longer periods of time :red_cross:
primary tissues consist of epidermis, cortex, vascular bundles, pith and leaves and create primary plant body :star:
present day woody plants derive from a single ancient species as the other two are extinct :first_place_medal:
Vascular Cambium
Ray initials
undergo periclinal cell division
one daughter cell becomes cambial ray initial
other becomes xylem parenchyma if inner cell or phloem parenchyma is outer cell
most significant difference is ray initials produce short cells mostly for storage parenchyma and in gymnosperms, albuminous cells
fusiform initials form
similar to fusiform initials except they are short and more/less cuboidal
arrangement of Cambial Cells
vascular cambium never has large regions of just fusiform initials or ray initials
if anticlinal divisions result in many fusiform initials side to side, central one may undergo transverse division and become ray initials
same is the opposite ray initial to fusiform and ratio is always constant and under precise developmental control
rays and fusiform initials are organized in specific patterns
Ray initials groupedin short vertical rows, 1 cell wide(uniseriate) 2 cell wide(biseriate) or many cells wide(multiseriate)
fusiform organized in horizontal rows (storied cambium) or irregularly without horizontal pattern (nonstoried cambium)
storied cambium evolved more recently than nonstoried in some advanced eudicots :star:
stored cambia tend to and selective advantage is unknown
single tree or shrub only has single vascular cambium that is
one cell thick
formed in a cylinder
extending upward toward trunk :+1:
outward into branches almost to tips of twigs :arrow_right:
downwards towards roots :-1:
each section forms as a narrow cylinder but is pushed outward by own production of wood cells on its inner surface :checkered_flag:
Fusiform initials
when undergoing longitudinal cell division with a wall parallel to circumference of cambium (periclinal wall)
produces 2 elongated cells
one continues to be fusiform initial
cambium cells produce narrow daughter cells all of which enlarge during differentiation
daughter cells located on inner side=mature secondary phloem, increase in diameter pushing cambium cells outward
cambuim is cylinder and outward movement results in larger circumference :red_circle:
vascular cambium cells must occasionally divide longitudinally by anticlinal walls, increasing # of cambium cells
without this, cambium cells would be stretched wider circumstantially and not function :warning:
one differentiates into cell of secondary xylem or secondary phloem
if outer daughter cell remains cambium cell, inner cell develops into secondary xylem
orientation is constant
wood never forms to the exterior of vascular cambium :red_cross:
bark never forms on interior side :green_cross:
one always remains cambium regardless of differentiation :star:
if inner cell remains cambium cell, outer cell develops into secondary phloem
not known how cells are picked for differentiation and cambium
but both xylem and phloem produced and always much more xylem than phloem
similar to apical meristems with thin primary walls and plastids present as proplastids.
after nuclear division
phragmoplast forms and elongates towards end of cell :straight_ruler:
phragmoplast can grow 50-100 micrometers per hour and divisions may take as long as 10 days :moon:
cell cycle may be as short as 19 hours in apical meristem cells of same plant
long, tapered cells with lengths of 140-462micrometers in dicots and 700-8700 micormeters in conifers :straight_ruler:
Initiation of the Vascular Cambium
in herbaceous species, cells between metaxylem and metaphloem of vascular bundle differentiate into conducting tissues
in woody species, cells in similar position never undergo cell cycle and continue to grow
This is called the fascicular cambium
cells that come out of the cell cycle arrest and resume mitosis form an interfascicular cambium that connects with fascicular cambium
term vascular and intervascular only used when cambium is young :star:
must be extended every year
tips of roots and stems initially only contain primary tissues
once metaxylem and metaphloem matured
that portion of root/stem contains both primary and secondary tissues
next growing season, apical meristem extends axis beyond this point
new segment of vascular cambium forms within and joins at base to top of vascular cambium from previous season :sunrise:
vascular cambium arises
vascular cambium in tree consists of segments of distinct ages (ground oldest, axis youngest) :timer_clock:
very rare that vascular cambium forms leaves that stay on for years :maple_leaf:
vascular cambium can never form flowers, fruits, or seeds
contains two types of cells, fusiform initials and ray initials
vascular cambium is one of the meristems that produce the secondary plant body
Secondary Xylem
growth rings
same time, new leaves formed and lose water at rapid rate causing a need for high capacity for conduction
called early wood/spring wood and contains high portion of wide vessels or wide tracheids :deciduous_tree:
transportation is less
wood produced here is called late wood/summer wood :sunny:
is larger and heavier and needs more mechanical strength
is much stronger due to numerous fibers or narrow, thick-walled tracheids
can have lower portion of vessels
will finally become dormant after this growing season is done :sleeping:
last growth is only heavy fibers with especially thick secondary walls
large number of newly formed vessels are conducting rapidly
later, cuticle has thickened
combination of the make up a years growth and is called an annual ring or growth ring :ring:
in regions with strong seasonal climates, vascular tissue is quiescent in times of stress, but when ceased, vascular cambium becomes active and cell division begins :recycle:
alternative arrangements exist where vessels form throughout the growing season :star:
wood growth with vessels throughout it is called diffuse porous
include yellow birch, aspen, and sugar maple
wood with vessels restricted mainly to early wood are ring porous
include red oak, sassafras, and honey locust
no notable difference in growth throughout seasons
in mild tropical climates, cambium can remain active almost continuously and hard to distinguish age, growth rings indistinct :sun_with_face:
Heartwood and Sapwood
moister outer region in center of log is known as sapwood
new layer of sapwood is formed each year by vascular cambium, one annual ring is then usually converted into heartwood :<3:
remains consistent in width with age :recycle:
functions for 10 years before being converted to heartwood :!:
different regions exist as vessels and tracheids do not function forever in water conduction. :check:
once broken, no means of pulling water upward, will never conduct water again :non-potable_water:
only few snap at a given time
new water filled tracheary elements are produced by cambium during next year
is important that vessel is wide for fungus to grow through easily
for vessel elements that are not conducting, mechanism that seals them off is selectively adventageous :check:
water columns can break due to multiple factors such as freezing and wind vibration
wood parenchyma cells adjacent to vessels push bubbles of protoplasm through pits into vessel forming plug called Tylosis
occurs repeatedly and the vessel may become filled
these and other wood parenchyma undergo numerous metabolic changes and produce large amounts of
phenolic compunds
lignin
dark-colored aromatic substances that inhibit bacteria and fungi growth :no_entry:
these chemicals are usually dark and aromatic, as they accumulate wood gets darker and more fragrant :nose:
all parenchyma die and conversion of sapwood to decay resistant heartwood is complete
dark wood in center of log is called heartwood
heartwood becomes wider with age :star:
types of wood cells
all cells formed interior of vascular cambium develop into secondary phloem and into wood
wood may contain
vessel elements
fibers
tracheids
sclereids
parenchyma
contains all types of cells that are in primary xylem but no new ones :older_man:
difference between primary and secondary xylem are origin and arrangement of cells :world_map:
secondary xylem reflects fusiform and ray initials
radial system develops from ray initials
in woody angiosperms, contains only parenchyma, arranged in uniseriate, biseriate, multiseriate masses called rays
conduct material over short distances radially within wood
2 basic types
upright cells
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procumbent cells
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ray parenchyma store carbohydrates and other nutrients during dormant periods
may also contain ray trachieds
they are horizontal, rectangular cells that look similar to parenchyma cells
contain secondary walls, cicular bordered pits, and protoplasts that degenerate quickly after secondary wall formed
In conifers, xylem rays are almost exclusively uniseriate
only multiseriste if they contain a resin canal
axial system derived from the fusiform initials
axial system always contains tracheary elements that carry out longitudinal conduction of H2O in wood :potable_water::
in woody angiosperms, contains fibers that gives wood strength and flexibility
hardwoods are woods with large amounts of fiber making them strong and useful for construction :hammer_and_wrench:
hardwood is term used for angiosperms and eudiots, even those that lack fibers or very soft like balsa :deciduous_tree:
wood form conifers(pines and redwoods), have few to no fibers and are called softwoods :evergreen_tree:
in many instances however they tend to be much harder than many hardwoods
tracheary elements and fibers elongate as fusiform initials produce them
immature cells undergo transverse divisions and differentiate into columns of xylem parenchyma
called axial xylem parenchyma and is important temporary reservoir of water :potable_water:
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complexity can vary
some angiosperms (basal) have mostly trachieds
majority of woody angiosperms contain all possible types of cells
water-storing parenchyma may be adjacent to vessels or arranged so that they never touch :potable_water:
fibers provide maximum strength when grouped in masses, if located around vessels
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most conifers contain only tracheids in axial systems, fibers and parenchyma absent
Reaction Wood
in angiosprems, reaction wood develops on upper side of branch, known as tension wood
cross section of this branch would show
tension wood contains special gelatinous fibers whose walls are rich in cellulose but little to no lignin
fibers exert tension, may even contract and slowly lift branch to more vertical orientation
eccentric growth rings being much wider on top: :star:
response to gravity, trees produce reaction wood
conifers form reaction wood on underside of branch :evergreen_tree:
rich in lignin and less in cellulose
growth rings especially wide on bottom side of limb
trunks or branches that are not vertical, gravity causes lateral stress=drooping and pendant if not counteracted
Secondary Phloem
has axial and radial system
axial system responsible for conduction up and down stem or root
contains sieve tube members and companion cells in angiosperms or sieve cells in conifers
both groups have fibers and nonconducting parenchyma in axial secondary phloem : :check:
may have fibers alternating with sieve tube members, not annual rings
no equivalent cells in secondary phloem compared to secondary xylem :red_cross:
size, shape and number of phloem rays match number of xylem rays , produced by same ray initials
consist of parenchyma cells used for storage :file_folder:
formed from vascular cambium just like secondary xylem
Outer Bark
Lenticels and oxygen diffusion
impermeability of cork has disadvantages
interferes with oxygen diffusion for inner wood e.g. sapwood and vascular cambium
keeps pathogens out and retains water but keeps out oxygen
Pathway for oxygen created when cork cells in bark become rounded as they mature
called lenticels
when new layer forms, same pathway is still there :world_map:
bark cells are more active here than adjacent cells leading to large build up of cells :house:
easy to spot in thin bark trees, thick tree lenticels are usually located at crack in bark near base
Cork and the Cork Cambium
Cork cambium/phellogen
all cells are cuboidal
inner cell will remain cork and outer cell will become cork cell/phellem cell after cell division and differentiation :star:
mature cells increase in size, encrusted in suberin, become waterproof and then die :non-potable_water:
cell death important, makes plant non-nutritious, waterproofing also kills cell layers outside of periderm due to lack of water
tissue outside innermost cork cambium=outer bark
amount of bark varies in species :scales:
secondary phloem between vascular cambium and innermost cork cambium = inner bark
formed by stretching and enlargement of circumference on periphery of plant
can produce another layer of parenchyma called phelloderm
layers are known as periderm
provides temporary protection as stem grows interior
layer of cork cannot expand much circumstantially
new layer will have to be formed after one or several years
will continue to act as protection for stem and prevent water/nutrients passage=build up of cork layers
is short lived :timer_clock:
Initiation of Cork Cambium
can be identified by color changes but can be years for cork to form in trees
growth in diameter is slow, occur in intervals, later cork cambium usually form deep in secondary phloem
initiation timing of cork more variable than vascular cambium :alarm_clock:
first cork cambium arises by reactivation of epidermal cells. first outer bark contains only periderm, and cuticle
if arises in cortex, first outer bark will contain periderm, cortex, and epidermis
as first bark sheds, cork cambia rises in secondary phloem to produce outer bark with cork and phloem only :check:
depends on type of cell types present in trapped secondary phloem
fiber cells produce fibrous stringy bark, sclereid- filled phloem produces hard bark :dark_sunglasses:
not unusual for young tree bark to be different than bark produced later in lifespan :recycle:
Secondary Growth in Roots
has same star shape as primary xylem but soon becomes round as more secondary xylem is produced in sinuses
some portions of cambium is pushed outward faster than others, when circular cambium is reached, unequal growth stops :busstop:
roots of conifers and woody angiosperms undergo secondary growth like stems
Root vascular cambium contains both ray and fusiform initials, and wood produced in root is similar to shoot :star:
roots have different conductivity requirements than stems
roots and shoot have differing roles and make up thus certain woody cells and functions are not similar despite same growth
however wood of roots are not identical to that of stems :silhouettes:
perennial roots also form bark
cork cambium produces cork cells that form protective layer and layer or two of phelloderm :house_buildings:
lenticels also occur being prominent near the lateral roots :left_right_arrow:
first cork cambium arises in pericycle causing endodermis, cortex, and epidermis to shed
several mechanisms exist by which storage capacity of woody root can increase
axial parenchyma also used and in many roots wood is pure parenchyma :diamonds:
orange part of carrot is wood parenchyma :carrot:
ray parenchyma of secondary phloem offer considerable volume and storage capacity increased as rays enlarge
Anomalous Forms of Growth
anomalous Secondary growth
development is fairly similar however, alternative types of cambium structures occur along with activity and arrangements
since cambium can differ from common types, it is called anomalous secondary growth
Included Phloem
after cambium stops dividing and begin differentiate into xylem, it is no longer cambium
begins producing ordinary secondary xylem and phloem then differentiating and ceasing to exist
this means xylem of second cambium is located exterior to phloem of first cambium
new vascular cambium will this arise in outermost second phloem and so on
phloem located between two bands of xylem is called included phloem :star:
cells in outermost, older secondary phloem are reactivated and differentiate into new vascular cambium
advantage of included phloem includes protection from insects. :check:
xylem and phloem have differing relationships than with normal xylem and phloem and advantages have yet to be studied : :question:
Secondary Growth in Monocots
a type of vascular cambium arises just outside the outermost vascular bundles
Originates from cortex cells in same matter as interfascular vascular cambium in eudicots :black_flag:
no monocots have secondary growth but become wood like like Joshua Trees and palms :red_flag:
this cambium produces only parenchyma, conducing cells completely absent
Columns of parenchyma undergo rapid division and produce narrow cells called secondary vascular bundles : :star:
contain xylem and phloem
outer cells of each bundle develop into fibers with thick secondary walls :recycle:
parenchyma that do not form like this form secondary ground tissue, identical to primary tissues
woody each year because of fibers, more conducting capacity and greater strength each year, makes branching feasible :check:
Roots of sweet potatoes
act normal except that xylem and phloem are produced purely by parenchyma. :red_flag:
new vessels may get surrounded by another new cambium and process repeats :recycle:
numerous vascular cambia arise around individual or groups of vessels
hundreds of cambia of different ages can be contained in single sweet potato
consist of secondary tissuse of irregular matrix parenchyma, few seive tubes, some vessels, and vascular cambia
amount of storage parenchyma increases due to anomalous growth in secondary growth :warning:
advantage of so many vascular cambia= large root grown quickly with large storage capacity
unequal activity of the Vascular Cambium
in ordinary growth, all areas of vascular growth is equal and therefor well rounded :black_flag:
however in some species there can be two active cambium sectors and 2 inactive sectors :red_flag:
advantage is related to flexibility
conduction increases and flexibility remains the same where other trees flexibility decreases :<3:
grows outward in two directions and thin in the other two
very advantageous for vines, is type of anomalous secondary growth that is adaptive
unusual Primary growth
appears hard because enclosed in strong fibers
for first years, tree becomes wider and leaves increase, no secondary growth occurs :warning:
vascular cambium never forms, wood and secondary phloem never occur
adventitious roots sprout from tree and add extra vascular bundle in process :heavy_plus_sign:
palm tree consists of all primary tissue with vascular bundles distributed throughout ground tissue :palm_tree:
bundles contain only primary xylem and phloem developed form shoot apical meristem
increase in adventitious roots along with increased with called establishment growth, from of primary growth. process will cease
conducting capacity set for life
same primary growth occurs in other monocots with extremely broad stems like corn :corn:
Palm trees are unusual in that their trunks do not tamper at tips and do not branch :star:
