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