Chapter 8
Vascular Cambium
Secondary Growth in Roots
Secondary Xylem/Phloem
Outer Bark
one of the meristems that produce secondary plant body
Interfasicular Cambium
Fasicular Cambium
mature parenchyma cells come out of cell cycle arrest, resume mitosis
connects fascicular cambia
cells b/t metaxylem & metaphloem never undergo cell arrest
must be extended each year
2 types of cells
fusiform initials: long, tapered cells
ray initials: short, slightly cuboidal
two elongate cells produced when longitudinal division occurs w/ periclinal wall
periclinal wall: wall parallel to circumference of cambium
one stays as fusiform initial, one differentiates
if outer cell remains cambium, inner develops into secondary xylem
if inner cell remains cambium, outer develops into secondary phloem
xylem cells increase greatly in diameter pushing cambial cells outward
have thin primary walls & proplastids
also periclinal division
one remains cambial cell, the other differentiates
inner cell: xylem parenchyma
outer cell: phloem parenchyma
mostly produce storage parenchyma & albuminous cells
Cambial Cell Arrangement
Fusiform:
can occur in storied rows, or unstoried rows
usually grouped in short vertical rows uniserate, biseriate, or multiseriate
uniseriate: one cell wide
biseriate: two cells wide
multiseriate: multiple cells wide
storied cambium: regular horizontal arrangement
unstoried cambium: irregular, no horizontal pattern
Xylem:
Phloem:
axial system: responsible for conduction up & down stem or root
all cells interior to vascular cambium develop into secondary xylem
aka wood
contain all cell types in primary xylem, no new
arrangment and origin differs from primary
axial system: derived from fusiform initials
radial system: develops from ray initials
always contains tracheary elements
hardwood: contain large amounts of fibers in axial system
term now used to describe all basal angiosperms & eudicots
softwood: all conifers, despite actual hardness
contains fibers for strength & flexibility
some immature tracheary elements & fibers undergo transverse division
differentiate into columns of xylem parenchyma
aka axial xylem parenchyma
crucial temporary water reserve system
cloudy/humid days water is moved into parenchyma
hot/dry days water drawn from parenchyma
contain only parenchyma in uniseriate, biseriate, or multiseriate masses called rays
ray parenchyma:
store carbohydrates & other nutrients during dormant periods
conduct material over short distances radially w/in wood
2 types of cells
upright cells
procumbent cells
no direct connection w/ axial cells
can have direct connection w/ axial cells
when adjacent to axial cell, plasmodesmata occur
starch within not digested until later
starch within digested first & passed to axial tracheary
in conifers
xylem rays almost exclusively uniseriate
multiseriate only if they contain a resin canal
ray trachieds: horizontal rectangular cells
look like parenchyma but have:
secondary cell wall
circular bordered pits
protoplasts that degenerate quickly after secondary wall complete
Growth Rings
early wood: first wood formed when quiescence ceases
must have high proportion of wide vessels
in conifers, wide tracheids
late wood: wood produced later in season
lower proportion of vessels, stronger w/ number fibers
in conifers, narrow, thick-walled trachieds
Heartwood: dark dry wood at center of log #heartwood/sapwood in root wood/shoot wood
made of broken vessels & trachieds
tylosis: plug of protoplasm pushed by adjacent parenchyma cells into vessels
Sapwood: lighter moist wood at outer region of log, becomes heartwood
Reaction Wood
produced in branches in response to stress
in angiosperms, occurs mostly on upper side of branch (known as tension wood)
tension wood: contains gelatinous fibers w/ walls rich in cellulose
exert tension on branch to prevent drooping
in conifers, occurs on underside of branch (known as compression wood)
enriched in lignin, less cellulose
contains sieve tube members & companion cells in angiosperms
contains sieve cells in conifers
Cork
storage parenchyma become reactivated & undergo cell division creating cork cambium #similar to activation of parenchyma cells during formation of interfasicular cambium
cork cambium: cells are cuboidal, inner cell always stays cambium, outer always differentiates into cork cell
aka phellem cell
aka phellogen
in some species, c. cambium creates layer of parenchyma cells on inside
aka phelloderm
all together compose the periderm
provides temporary protection due to continuous interior growth stretching it circumferentially
outer bark: all tissue outside innermost cork cambium
inner bark: all secondary phloem b/t vasc. cambium & innermost cork cambium
lenticels: regions of aerenchymatous cork that aid in oxygen diffusion
Cork Cambia Initiation
timing of initiation varies by species
origin in epidermal cells: first outer bark contains only periderm & cuticle, very smooth
origin in cortex: first outer bark contains periderm, cortex, & epidermis, also smooth
origin in phloem: later bark, contains only cork and phloem
new vascular cambium retains star shape of primary xylem but soon becomes rounded
unequal growth ends when vasc. cambium becomes circular
root wood similar to shoot wood, has heartwood & sapwood
Anomalous Forms of Growth
anomalous secondary growth
growth of alternative types of cambium structure/activity
sweet potatoes:
storage parenchyma increased dramatically
numerous vasc. cambium arise not around entire primary xylem but around vessels/vessel groups
cambia after normally, except xylem/phloem produced almost purely parenchyma
new vessels surrounded by new cambium
some eudicots produce included phloem
common vasc. cambium produces ordinary secondary xylem & phloem then differentiates into xylem
outermost secondary phloem reactivated & become new vasc. cambium, produces secondary phloem b/t two bands of xylem
unusual primary growth
palm trees:
trunk all primary tissue w/ vasc. bundles disturbed through ground tissue
bundle contains only primary xylem & phloem
vasc. cambium never develops
establishment growth: increase in width & addition of adventitious roots