Download Stems and leaves

Stems and leaves
• All part of a dynamic shoot system
• Stem Functions:
– Support
– Produce carbohydrates
– Storage
– Transport between roots and leaves
• Growth
– At nodes and internodes
– Buds = apical and axillary
– Intercalary meristems (base of nodes and
leaf sheaths)
Rosette plants = plants where the stem
does not elongate
The Structure of Stems
• 3 tissues
– (1) Epidermal
– (2) Ground
– (3) Vascular
Epidermal Tissue:
– One cell thick, transparent, has trichomes
– Covers stems until turns into bark
Vascular Tissue:
– Arranged relative to leaves
– Xylem and phloem occur in Vascular Bundles !!
Typical Vascular Bundle
• Phloem forms first,
on outside
• Xylem second, on
inside
• Enclosed or found
with sclerenchyma
fibers
• Procambium
(meristematic)
• Monocot vs. Dicot
Monocot Stem Cross Section
Lack vascular cambium in mature forms. Vascular bundles embedded throughout
tissue.
Dicot Stem Cross Section
Secondary growth, single ring of vascular bundles embedded in ground tissue.
Ground Tissue:
– Cortex (monocots and dicots) and pith (dicots)
Axillary Buds and Branching:
– Axils
– Meristematic but dormant normally
– Apical Dominance
Some dicots and nonflowering plants:
– Concentric cylinders of xylem and phloem
Modified Stems
• That grow underground
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Bulbs
Rhizomes
Corms
Tubers
• That grow above ground
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Runners
Tendrils
Searchers
Cladodes
Thorns
Succulents
The Leaf
How leaves form:
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Leaf buttress or leaf primordia (100-300 cells)
The apical peg, apical meristem and midrib
Adaxial side grows slower than the abaxial
Unequal growth causes early leaf to arch over apical
meristem, protecting it
• Adaxial meristem forms, thickens leaf
• Eventually leaf forms upper leaf zone and lower leaf
zone
• Upper zone contains marginal meristems to broaden
the blade and form petiole
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Upper leaf zone flattened
Lower leaf zone forms leaf base
Leaf approximately 6 cells thick
Continued leaf growth by cellular expansion and
division along margins