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