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