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Lecture 7: Stem modification (Annual rings (Uses (Age determined by…
Lecture 7: Stem modification
Annual rings
Forms when temperate climates with a growing period and a period of dormancy (during winter)
Or in areas with pronounced wet dry seasons
Uses
Age determined by counting the annual rings
Understand past climate
Determine growth rate of tree
Tree soundness
Penetration of chemicals in wood treating business
Enviromental Science: to study the effects of air pollution, lighting, etc.
Geology: To locate the epicentres and estimate magnitudes of old earthquakes
Formed by the vascular cambium
In each growing season,new secondary xylem and phloem are produced. Old xylem cells eventually clog and no longer transport sap, turning into heartwood.
Type of wood
Light wood
(Tree makes it in the early spring) (Rings)
Dark wood
(Tree makes it in late summer) (Rings)
Heartwood
Older, non-living wood in the center of a woody plant
Usually harder, darker in color than younger sapwood
Sapwood
Outer wood formed in the cambium of a tree trunk
Active in transport of water
Usually lighter in color than hardwood
Knots
Simply branches that the tree has either grown over or around
Knots make whole pieces of wood weaker
Bark
Outermost layer of the tree trunk
Produced by cork cambium
Stem modification
Purposes
Protection
Climbing
Storage
Asexual reproduction/ Vegetative reproduction
Single parent, offspring similar
A single individual may split, bud or fragment to give rise to new individuals
Underground stem modification
Rhizomes
Horizontal, swollen underground stem
Presence of scale-like leaves, buds, nodes and internodes
Scaly leaves have resting buds in the axils
Propagated by division, when these resting buds will grow and produce leaves for a new plant
Tubers
Fleshy underground stems enlarged for storage
E.g. Potatoes
Bulb
An underground storage organ formed from the plant stem and leaves
E.g. Lilies, tulips, onions
Corm
Short, thickened underground stem, usually covered with papery leaves
E.g. Crocus, gladiolus, cyclamen, water chestnut
Sub-aerial Modification of stems
Runners
Horizontal above ground stems with long internodes creeping on the ground and rooting at the nodes
Arise as axillary bud, and creeps some distance from mother plant before rooting
Stolons
Slender stem that grows horizontally along the ground, giving rise to roots and aerial branches at nodes
Similar to runners but is underground
Buds develops along stolons and give rise to new plants
Sucker
A lateral branch of an underground stem but grows upwards into a new plant
E.g. Pineapple, Banana, etc.
Aerial
Stem tendrils
Leafless, spirally coiled branches
Helps weak stems to climb
Modification can be of an axilary bud
Thorn
Phylloclade
Green commonly flattened or sometimes rounded stem or branch which performs the functions of leaves
Cacti