Plant Secondary Growth: It’s Better Than Bad, It’s Good! Whaddya’ know?
1) What’s the difference between primary growth and secondary growth?
2) What does secondary growth give rise to (i.e., what do we usually call secondary growth?)
3) What are the evolutionary advantages to secondary growth for the plant?
4) What are the advantages to secondary growth in the ecosystem? Primary growth: Review • Builds the primary plant body from cell divisions generated in the apical meristems - dermal - vascular tissues - ground
• Responsible for lengthening roots and shoots
• Differs between roots and shoots
• Accounts for nearly all of the growth we see in annual, and herbaceous plants
• Most metabolic activity happens in parts of the plant body undergoing primary growth Shoot Apical Meristem Leaf primordia Primary growth - Shoots • Origin of three tissue systems from the Shoot Apical Meristem (SAM)
• Leaves develop from leaf primordia; internodes are short
• Proximity to the SAM can suppress growth of the axillary Developing bud meristems vascular - Can you think of how that strands might happen?
Axillary bud • Apical dominance give trees, meristems like gymnosperms, their conical LE 35-15 0.25 mm shape Primary growth - Roots Cortex Vascular cylinder Epidermis • Origin of three tissue systems Key Zone of from the Root Dermal Root hair maturation Apical Ground Meristem Vascular (RAM)
• RAM is Zone of bifacial elongation
• Lateral roots (branch Apical Zone of cell analogs) meristem develop from division the pericycle. Root cap
100 µm LE 35-12 Meristems are areas of perennial juvenility • Meristematic cells divide relatively frequently.
• Some of the new cells stay in the meristem (initials), others (derivatives) differentiate to form, and increase the size of the plant body.
• Apical meristems are responsible for primary growth Shoot Apical Meristems (SAM) Axillary bud meristems (pieces of SAM) Root Apical Meristems (RAM)
• Lateral meristems are responsible for secondary growth Vascular cambium Cork cambium Residual procambium and pericycle (in the roots) Meristems are areas of perennial juvenility Shoot apical meristems Primary growth in stems Epidermis Cortex
Primary phloem
Primary xylem Vascular cambium Lateral Pith Cork cambium meristems Secondary growth in stems Periderm Cork Pith cambium Cortex Primary Primary xylem phloem Secondary xylem Secondary Root apical meristems phloem Vascular cambium LE 35-10 Secondary growth adds girth • Responsible for increasing girth (increase in size around) in roots and shoots of some herbaceous annuals, but mostly perennial plants
• Results from activity of the lateral meristems in the stems (and roots) of gymnosperms and eudicots (but not monocots!)
Secondary growth in stems -vascular cambium adds 2º xylem (wood) and 2º phloem -cork cambium produces periderm (bark)
Secondary growth in roots - vascular cambium/residual procambium adds 2º xylem (wood) and 2º phloem - pericycle produces periderm
• Rarely see 2º growth in monocots (grasses, palms, orchids, lilies, etc.) Primary and secondary growth in a two-year-old stem
PRIMARY Epidermis Cortex Pith Primary xylem Primary phloem Vascular cambium Primary phloem Vascular cambium Cortex Epidermis Primary xylem Phloem ray Pith Xylem ray
Primary xylem
Secondary xylem SECONDARY Vascular cambium Secondary phloem Primary phloem Periderm First cork cambium Cork (mainly cork cambia and cork)
Primary phloem
Secondary phloem Secondary xylem (two Vascular cambium years of production)
Secondary xylem Vascular cambium Primary xylem Secondary phloem Bark Most recent cork Layers of Pith cambium Cork periderm LE 35-18a LE 35-18b Secondary growth adds girth Secondary phloem – consists of sieve tube elements, parenchyma and companion cells Vascular cambium Late wood Cork Secondary cambium Periderm xylem Early wood Cork
Transverse section of a three-year- Xylem ray – old Tilia (linden) Consists of parenchyma stem (LM) cells and tracheids in Bark Horizontal orientation 0.5 mm 0.5 mm
Secondary xylem Secondary phloem LE 35-20
Growth ring
Vascular ray
Heartwood Secondary xylem Sapwood
Vascular cambium
Secondary phloem Bark Layers of periderm Fig. 35-UN3 Whaddya’ know now?
1) What’s the difference between primary growth and secondary growth?
2) What does secondary growth give rise to (i.e., what do we usually call secondary growth?)
3) What are the evolutionary advantages to secondary growth for the plant?
4) What are the advantages to secondary growth in the ecosystem? Whaddya’ know now? 1) What’s the difference between primary growth and secondary growth? primary growth increases the length of roots and shoots giving rise to the primary plant body, whereas secondary growth increases the girth (circumference) of shoots and roots (giving rise to the secondary plant body). Secondary growth is most prevalent in long-lived perennial plants. Whaddya’ know? 2) What does secondary growth give rise to (i.e., what do we usually call secondary growth?) Wood! (wood and bark to be more precise!) Whaddya’ know? 3) What are the evolutionary advantages to secondary growth for the plant? Allows plants to grow tall because they have supportive structural tissue. Height results in better competitive advantage for light. Better access to light means more leaves, higher rates of photosynthesis, more energy to allocate to reproduction. Whaddya’ know? 4) What are the advantages to secondary growth in the ecosystem? Trees provide homes and food for lots of critters. Trees sequester TONS of carbon for long periods of time! Shoot Apical Meristem Leaf primordia Primary growth - Shoots • Origin of three tissue systems from the Shoot Apical Meristem (SAM)
• Leaves develop from leaf primordia; internodes are short
• Proximity to the SAM can suppress growth of the axillary Developing bud meristems vascular - Can you think of how that strands might happen?
Axillary bud • Apical dominance give trees, meristems like gymnosperms, their conical LE 35-15 0.25 mm shape