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Unformatted text preview: Phloem Differentiation, Time Secondary Growth in Plants
Woody plants (trees & shrubs).
Thickening of older, woody stems &
Only in preexisting plant parts.
2 secondary meristems involved:
adds new woody layer to stem
forms outer covering (periderm) &
epidermis. Cells & Tissues of the Plant Body
Parenchyma Cells: (p. 541; 513-514)
• Often in continuous masses
• Living at maturity.
• Only a primary cell wall.
• Basic metabolism, photosynthesis,
storage, secretion & transport.
• Most numerous cell type
• Retain ability to divide & originate new
meristems Transfer Cells (p. 542; 514)
Parenchyma cells with wall ingrowths to increase
membrane surface area.
High-volume short-distance solute movement
Often adjacent to vascular tissues, etc. Collenchyma Cells (p. 542-543; 514)
Unevenly thickened non-lignified, flexible
Support soft, growing tissues.
Living at maturity.
Occur as strands of collenchyma tissue.
• “Strings” in celery. Sclerenchyma Cells (p. 543-544; 514-516)
Mechanical cells: hard, rigid secondary cell wall, dead at
– Sclerids: short branched; fruits, nuts, etc.
– Fibers: long, in strands; mature parts of all plants.
Occur both in-mass (sclerenchyma tissue) & mixed with
other cell types.
other Fibers Sclerids Xylem: (p. 544-546; 516-518) (Part 1 of the Vascular tissue system)
Conducts water & soil minerals.
Two possible origins:
• root & shoot apical meristem (primary growth) =
• From vascular cambium (secondary growth) =
secondary xylem = wood.
Complex tissue (many cell types). Types of Cells in xylem: (p. 544-545; 517-518; chart p. 548; 519)
• Small diameter conducting cells
• All vascular plant groups.
• Larger diameter, flat perforated ends.
• Flowering plants only.
Sclerenchyma fibers: strength. Tracheid Vessel Phloem (p. 546-553; 519-523)
Principal food conducting tissue of vascular plants:
• Transports sugars (sucrose).
• Other substances: Amino acids, hormones, lipids,
Complex tissue: more than 1 cell type. Phloem Cell Types (p. 543; 519 (Table 23-1)
Sieve elements-- Conducting cells:
Living cytoplasm but no nucleus
• Sieve cells (gymnosperms).
• Sieve tube members (have sieve
plates), flowering plants.
Specialized parenchyma helper cells:
• Albuminous cells (gymnosperms).
• Companion cells (flowering plants).
Other parenchyma cells.
Sclerids & fibers. Sieve tube Companion
cell Dermal Tissues:
Epidermis (p. 553-555; 523)
Covers/ protects non-woody plant parts.
Origin: apical meristems (primary...
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