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Secondary Growth
Tree Rings
• Rings visible in heartwood and sapwood are regions of early
and late wood
• Early wood forms during wet springs
• Late wood indicates a dry summer or drought when no large-
diameter xylem cells were made for water uptake
• In most temperate zone trees, one ring forms each year
Fig. 25.18b, p. 409
B Early and late wood in an ash tree. Early wood forms
during wet springs. Late wood indicates that a tree did not
waste energy making large-diameter xylem cells for water
uptake during a dry summer or drought.
direction of growthvessel in xylem
early early early early earlylatelatelatelate
Early Wood and Late Wood
• Secondary Growth
• In many plants, secondary growth thickens branches and
roots during successive growing seasons
• Extensive secondary growth of eudicots and conifers
produces wood
• Tree rings can be used to study past environmental
conditions
Variations on a Stem
• Specialized stems allow some plants to store nutrients, to
reproduce sexually or asexually
• Specializations include stolons, rhizomes, bulbs, corms,
tubers, and cladodes
Stolons
• Stolons (runners) are
stems that branch from
the main plant stem
• Adventitious roots and
leafy shoots sprout from
nodes and develop into
new plants
• Example: strawberry
Rhizomes
• Rhizomes are fleshy,
primary stems that grow
under the soil, parallel
to its surface
• They are the plant’s
primary storage tissue
• Example: turmeric
Bulbs
• A bulb is a short
underground stem with
overlapping layers of
thick, modified leaves
(scales)
• Contains starch and
other stored products
• Example: onion
Corms
• A corm is a thickened
underground stem that
stores nutrients
• Unlike a bulb, a corm is
solid rather than layered
• Example: taro
Tubers
• Tubers are thickened
portions of underground
stolons
• They are the plant’s
primary storage tissue
• Example: potato
Cladodes
• Cladodes are flattened,
fleshy, photosynthetic
stems that store water
• Example: cactuses
Key Concepts
• Modified Stems
• Certain types of stem specializations are adaptations for
storing water or nutrients, and for reproduction
Tree Rings and Old Secrets
• Many trees form one ring each year
• Tree rings hold information about environmental conditions
that prevailed while the rings were forming
• Example: Relative thicknesses of rings reflect the availability
of water – rings show that settlers who arrived at Roanoke
Island in 1587 suffered a major drought
A Record of Rainfall
• A section of a bald cypress tree that was living near English
colonists when they first settled in North America – narrower
annual rings mark years of severe drought
Tree Rings and Old Secrets
• Tree rings are used to
date archaeological
ruins; gather evidence
of wildfires, floods,
landslides, and glacier
movements; and study
the ecology and effects
of parasitic insect
populations
p. 411
2year: 1 3Tree Rings
and Old
Secrets
Some Tree Rings
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