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