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

Early Wood and Late Wood vessel in xylem direction of growth early late early late early late early late early 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. Fig. 25.18b, p. 409

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

Tree Rings and Old Secrets year: 1 2 3 p. 411

Some Tree Rings