stems purpose of stems support leaves transport water and nutrients store water and food
TRANSCRIPT
STEMS
Purpose of Stems
•Support leaves
•Transport water and nutrients
•Store water and food
WHY?!?•Growth in height only
occurs at tip of roots and branches
Specialized Stems
• Cactus– is a stem that
stores food and water for the plant
– spines are leaves of a cactus
• Rhizomes– underground stem
which grows horizontally through soil
– Ex. Iris
Specialized Stems
• Bulbs– specialized stems
that store food– Ex. onions, tulips,
daffodils
Specialized Stems
• Corms– Short, swollen
underground plant stem used as food storage
– Ex. crocuses, gladiolas
• Stolons– slender stem that
grows above ground
– “touch, root-and-shoot”
– Ex. strawberries
Specialized Stems
Where do Stems Grow?
• Apical Meristems– cell division
occurs at tip of stem
• Lateral Meristems– cell division
occurs – stems grow in
diameter
Structure of stems
• Nodes– region on stem
where 1 or more new leaves form
• Internode– distance between
segments
• Lenticels– pores in surface of
stem– allow gas exchange
• Buds– Terminal - bud at
the end of stem– Lateral - buds on
side of stem– Bud scales-
protective covering over embryonic shoots (present in winter)
Structure of stems
Lateral Bud
Primary Growth in Stems
• Epidermis– outer layer– protection– prevents H20 loss– contains lenticels
• allow for O2 and CO2 exchange
• Cortex– lies inside epidermis– storage of food for stem
Primary Growth in Stems
• Pith– located in center
of stem– stores food
• Vascular Bundles– Xylem : transfers
H2O– Phloem : transfers
food
Type of stems
• Monocot Stem– **V.B. are
scattered**– xylem in center– phloem on outside
Types of Stems
• Dicot Stem– **V.B. make a
circle**– xylem closer to
center– phloem behind
xylem
Secondary Growth in Stems
• Occurs mainly in dicots b/c monocots lack lateral meristems
• Stems increase in diameter due to lateral meristems– 2 types : vascular
cambium, cork cambium
• Vascular cambium– makes new
xylem, phloem through cell division
– this becomes secondary xylem & secondary phloem
Secondary Growth in Stems
• Wood– is secondary
xylem
• Heartwood (pith)– is older primary
xylem– stopped
transporting H20
Secondary Growth in Stems
Secondary Growth in Stems
• Sapwood– new secondary
xylem– lighter in color (still
transports H20)
• Bark– protection for woody
stems– made up of cork,
cork cambium, phloem
Why does bark appear to be rough or crack?
• Cells aren’t living– cork cambium
produces cork, but dies before maturity
– as tree grows, cork ruptures and forms cracks
Annual rings• Springwood
– xylem produced in spring
– rain is plentiful– cells are larger than
summer wood
• Summerwood– xylem still produced– limited amt. of
water– cells are smaller
than springwood
Annual rings
• Annual rings– during hibernation
xylem isn’t produced– difference from 1 year
to the next– can tell app. age of
tree– tell environmental
conditions– lg.= moist– sm. = drought