Transcript Plant Growth Basics

Stems and Plant
Growth Basics
Shoots vs. Roots
• Stems are part of the shoot system (stems,
leaves, flowers)
• The shoot system depends on the roots for
water and minerals, and to store excess sugar
created by photosynthesis
Anatomy of a Stem
• Node: area where leaf branches off from stem
• Internode: area in between nodes
• Apical meristem: terminal bud where upward
growth in concentrated
• Axillary bud: site of new
branches
• Petiole: area connected leaf to
stem
Anatomy of a Stem
• Leaf scar: area where leaf has fallen from stem
• Terminal bud scar: remnant of previous year’s
terminal bud; marks one year of growth
Terminal vs. Axillary Buds
• The growth of young shoots is concentrated at
the terminal bud, where cells are dividing
rapidly
• The terminal bud is dominant over the axillary
buds in young plants, keeping them dormant
• We can prune/pinch plants to influence shape
– Trimming terminal bud increases width
– Trimming axillary buds increases height
Inside a Stem
• Young stems are made of xylem, phloem, a
pith, vascular cambium, and epidermis
– Xylem: transports water from roots to leaves
– Phloem: transports sugar from leaves to roots
– Pith: center of stem; functions in storage
– Vascular cambium: layer of cells that makes new
xylem and phloem
– Epidermis: protective coating
Inside a Stem
• Dicots have a large central pith, with vascular
bundles around the edges (a vascular cylinder)
• Monocots have vascular bundles randomly
dispersed throughout the pith
Why do plants grow forever?
• Plants can grow throughout their lives
because of meristems
• Meristems are bundles of stem cells that make
new cells throughout a plant’s life
– Apical meristems: responsible for primary plant
growth (increase in height)
– Lateral meristems: responsible for secondary plant
growth (increase in diameter)
Primary Plant Growth
• Apical meristems are located at the tips of
roots and stems
• Apical meristems give rise to the epidermis,
vascular tissue (transport), and ground tissue
(storage)
Secondary Plant Growth
• Secondary growth increases plant diameter
• Lateral meristems cause secondary growth
– Vascular cambium
– Cork cambium
Vascular Cambium
• The vascular cambium is located between
layers of xylem and phloem
• The vascular cambium makes secondary xylem
and phloem (wood and bark)
Cork Cambium
• The cork cambium
makes cork, a tough
outer coating that
replaces the epidermis
on stems and roots
• This is what we think of
as ‘bark’, but technically
bark includes secondary
phloem, the cork
cambium, and cork