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STEMS
TYPES OF STEMS
Either woody or nonwoody
Generally, adapted to support leaves; also
transport materials and provide storage
-stolons – grow along soil surface and produce
new plants (strawberry)
-tubers – enlarged, short, underground stems
used for storing starch (potato)
-succulent – fleshy, water-storing stems
(cactus)
PRIMARY GROWTH
Stems grow in length at tips (apical meristem) and in
diameter (lateral meristem)
Each leaf is attached to the stem at a node.
The spaces between nodes are internodes.
Bud – apical meristem capable of developing into a
new shoot system, enclosed by specialized leaves
(bud scales)
- lateral – located at nodes
- terminal – located at tips of stems
PRIMARY GROWTH
Dermal tissue – represented by epidermis (protect
plant and reduce water loss while allowing gas
exchange)
Ground tissue – pith and cortex in dicots and
gymnosperms, cortex only in monocots
Vascular tissue – each bundle contains xylem
(towards inside) and phloem (towards outside)
-monocots – scattered throughout stems (monkey
faces)
-dicots and gymnosperms – single ring, with
secondary tissues replacing primary tissues
SECONDARY GROWTH
Stems increase in width (thickness) due to
division of cells in the vascular cambium
(lateral meristem)
More secondary xylem is produced than
secondary phloem
-heartwood – darker wood in the center that no
longer transport water
-sapwood – functional, lighter-colored wood
nearer the outside of the stem
SECONDARY GROWTH
Bark – protective outside covering of woody plants
(cork, cork cambium, phloem)
-cork cells are dead, and so rupture as the stem
continues to expand (textured bark)
Springwood – wide, thin-walled xylem produced
when water is plentiful
Summerwood – smaller, thicker-walled xylem
produced when water is limited
Annual ring – abrupt change between springwood
and summerwood; used to estimate age of plant
STEM FUNCTIONS
Transport nutrients and water
-cohesion-tension theory – cohesion and
adhesion allow the continuous column of water
to move up plant
- transpiration (water loss through stomata)
occurs while plant exchanges gases
Storing water and nutrients
-occurs in parenchyma cells in cortex
STEM FUNCTIONS
Support of leaves
Translocation – movement of carbohydrates
through the plant, from source (where the
carbs are made) to sink (where the carbs are
used); water follows the movement of carbs,
causing pressure which keeps the carbs
moving