Bellwork: What is the difference between primary and secondary growth of stems? How are they related? Why does this result in certain plants being able to grow taller?

Stems Section 23.3

What are the 3 main functions of stems? Produce leaves, branches and flowers Hold leaves up to the sun Stems transport substances through the plant

Stem anatomy Stems contain all three tissue types – dermal, vascular and ground tissue Layer of epidermal cells that have thick cell walls and a waxy protective coating Node – where leaf is attached to the stem Bud – contains apical meristem that can produce new stems and leaves Stem can develop woody tissues that help support leaves and flowers

Vascular bundle patterns Monocots have clusters of xylem and phloem tissues called vascular tissues In dicots vascular bundles are arranged in a cylinder or a ring

Plant stem growth Primary vs Secondary growth Primary growth - of new cells produced by the apical meristems of roots and stems Increase in length in a plant is due to primary growth from year to year Primary growth of stems is the result of elongation of cells produced in the apical meristem Occurs in all seed plants Secondary growth – Stems must increase in thickness as well as length if the plant gets big and have more mass to support Common in dicots and non flowering seed plants such as pines Rare in monocots Explains why most monocots have limited girth

Secondary growth continued… Most dicots have meristems within their stems and roots Enable true secondary growth Increased width is able to support increased height of tree In conifers and Dicots, secondary growth occurs in meristems called the vascular cambium and the cork cambium Vascular cambium – produces vascular tissue and increases stem thickness over time Cork cambium – produces the outer covering of stems Similar types of cambium are also found in roots]

Growth from the Vascular Cambium In a young dicot stem, bundles of xylem and phloem are arranged in a ring Secondary growth begins as a thin, cylindrical layer of cells between clusters of vascular tissue New meristem forms between the xylem and the phloem of each vascular bundle Division of vascular cambium gives rise to new layers of the xylem and phloem Each year the cambium continues to produce new layers of vascular tissue, causing the stem to become thicker and thicker.

How does wood form? Most of what we call wood is layers of secondary xylem produced by the vascular cambium These cells build up year after year, layer upon layer Older xylem – heartwood – found in the center no longer conducts water Darkens with age as it collects colored deposits Sapwood surround heartwood – is lighter in color, and is active in fluid transport

What is a tree ring? How does it form? Tree growth is seasonal in temperate zones When growth begins in the spring, the vascular cambium begins to grow rapidly, producing large, light colored xylem cells with thin cell walls Results in light colored earthy wood As growing season continues, cells grow less and have thicker cell walls, forming a layer of dark late wood Alternation of dark and light wood produces tree rings Usually one ring is a year of growth Tree ring cross sections can be used to estimate a trees age Size of rings can provide additional information about weather conditions

How does Bark form? In a mature stem all of the tissue outside of the vascular cambium makes up bark Includes Phloem, Cork Cambium and cork As a tree expands in width, the phloem layer must grow as well Expansion may cause the oldest tissues to split and fragment as the expanding stem stretches them The cork cambium surrounds the cortex and produces a thick , protective layer of waterproof cork that prevents water loss from the stem As stem increases in size, outer layers of dead bark often crack and flake off the tree

Key points questions What are three important functions of a stem? How does the arrangement of vascular bundles differ between monocots and dicots? How do the functions of a stem relate to the functions of roots ad leaves? Define primary and secondary growth