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Tissues and the Primary Growth of Stems (External Organization of Stems…
Tissues and the Primary Growth of Stems
Cell and Tissue Types
Sclerenchyma
walls are elastic
dead at maturity
primary wall and thick secondary wall
water transport
provide elastic support
long fibers vs. short sclereids
storing starch or crystals of calcium
pits = narrow & in secondary wall
pit + pit = pit-pair
Collenchyma
present in elongating shoot tips
require more glucose
walls thin and thick depending on area
alive at maturity
uneven thickness
provide plastic support
Parenchyma
primary walls that remain thin
tissue = mass of cells
chlorenchyma = cells in photosynthesis
glandular cells = secrete
transfer cells = short distance transport
alive at maturity
External Organization of Stems
stem area above node = leaf axil
internodes = regions between nodes
axillary bud = miniature shoot
nodes = where leaves are attached
bud scales = modified leaves that protect
terminal bud = extreme tip
stolons = runners
bulbs = short shoots, thick fleshy leaves
phyllotaxy = arrangement of leaves :leaves:
opposite = leaves two per node
decussate = leaves located in four rows
alternate = leaves one per node
whorled = three or more leaves per node
spiral = leaves not aligned with nearest neighbors
distichous = leaves located in two rows only
corms = vertical, thick stems & papery thin leaves
rhizomes = fleshy horizontal stems
tubers = like rhizomes but grow for short periods
Internal Organization of Stems
Vascular Tissues
phloem
sieve tube = large sieve areas aligned
sieve plates = end-wall sieve areas with large sieve pores
sieve pores = plasmodesmata enlarge
albuminous cells = associated with sieve cells
sieve element = sieve cells & sieve tube members
companion cells = control sieve tube members
xylem
annular thickenings = secondary wall in set of rings :ring:
helical thickening = 1-3 helices interior to the primary wall
sclerenchyma = tracheids + vessel elements
Scalariform thickening = provides strength :muscle:
conducting cells
reticulate thickening = secondary wall, net shape
circular bordered pits = derived and strongest tracheary elements
border = makes pits stronger
pit membrane = primary walls and middle lamella between pit pairs
perforation = large hole
vessel = stack of vessel elements
Vascular Bundles
pith = region of parenchyma
xylem + phloem; run parallel to each other
Cortex
photosynthetic parenchyma
usually fits compactly
interior to epidermis
Epidermis
prevent water loss :droplet:
shields internal cells
the outermost surface of an herbaceous stem
thin walled parenchyma cells
cutin = fatty substance makes walls impermeable to water :umbrella_with_rain_drops:
cuticle = layer of cutin
stoma = guard cells + stomatal pore :guardsman::skin-tone-2:
trichomes = hairs :person_with_blond_hair::skin-tone-2:
Stem Growth and Differentiation :sunflower:
metaxylem = longest time of growth ---> largest tracheary elements
subapical meristem = just below apical
protophloem = mature exterior cells
apical meristems = stem grows longer by new cwlls
metaphloem = cells closest to metaxylem
protoderm = epidermal cells in early stages
provascular tissues = young phloem and xylem
ground meristem = young stages of pith and cortex