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 H 2 0 loss –contains lenticels allow for O 2 and CO 2 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 H 2 O –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 H 2 0 Secondary Growth in Stems

Sapwood –new secondary xylem –lighter in color (still transports H 2 0) 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 –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