Stem Lecture

Support of Leaves, Flowers, Fruits

Conduction of Water, Minerals, Sugars, etc.

PhotosynthesisStorageDefense

Functions

Stems support a display of leaves.

Stems orient the leaves toward the light with minimal overlap among the leaves.

Asclepias - milkweed

Cercis canadensis - redbudThe stem supports a display of flowers

Ipomoea nil - morning glory

The stem supports a display of fruits.

The stem of a vine “twines” around objects in the environment - circumnutation!

The stem does photosynthesis…and stores water.Opuntia-prickly pear

This stem does photosynthesis, stores water, but also produces a defense chemical: mescaline…a hallucinogen.

Lophophora williamsii - peyote

Stem Lecture

Support of Leaves, Flowers, Fruits

Conduction of Water, Minerals, Sugars, etc.

PhotosynthesisStorageDefense

Structure to Provide Functions

Typical Stem Cross SectionHelianthus annuus-sun flower annual

Pith

A ring of vascular bundles

Epidermis

Cortex

Epidermis - window, reduce

water lossCortex Collenchyma- extensible support

Cortex Parenchyma- photosynthesis,

etc.Phloem Fibers- rigid support

Functional Phloem- conduct sugars etc.

away from leaf to rest of plantVascular Cambium

- adds 2° xylem and 2° phloemXylem

-conduct water and minerals

up from soilPith-water storage,

defense?

VIP Stem: Provide both name and function labels:

outside

to center

Epidermis: reduce evaporation, gas exchange

Cortex: photosynthesis, collenchyma support

Vascular Bundles: conduction

Pith: water storage? defense? disintegrate?

Vascular Bundle:

Phloem Fibers: supportFunctional Phloem: conduct CH2O away from

leafVascular Cambium:

add 2° Xylem and 2° PhloemXylem:

conduct minerals up from soil

outsi

de

to

center

Vitis vinifera - grape

Vitis vinifera - grape

Notice how the vascular cambia of adjacent vascular bundles line up side by side.Notice that cambium tissue differentiates between the bundles, connecting the cambia together.

Vitis vinifera - grapeThe vascular cambium makes 2° tissues:

2° phloem2° xylem

Each year the cambium produces a layer of secondary xylem and a layer of secondary phloem.This photo shows secondary xylem from parts of three years in Pinus strobus (white pine).

mid-summer of one year

fall of that yearwinter of that yearspring of the next year

Three years of Secondary Growth Tilia - basswood

SecondaryXylem

Secondary Phloemcamb

ium

The study of the growth rings in wood: Dendrochronology

This tree is Pinus aristata (bristlecone pine).One individual of this species shows more than 5000 growth rings! Inner wood, harvested by boring, was used to validate carbon-14 dating. Imagine the stories that this California tree could tell…perhaps something of migration of Asian peoples down the western coast of North America! They were contemporaries of Pharaohs!

The epidermis will be stretched and torn if not replaced

Sambucus canadensis - elderberry

A cork cambium differentiates and produces a periderm.

Epidermis

Cork Cambium

Phelloderm

cutin

suberin

Cork Cells

Over time, the epidermis dies.

The cork cells build up to for a thick layer for the bark of a tree. We use this to make stoppers for wine bottles and so on.When suberin is fully developed, the cortex cells will eventually be in the dark. So these chloroplasts will lose their function!

Sequoia sempervirens - giant sequoia

The thick periderm can be quite thick and assist in survival of forest fires!

Randy is about six-feet tall!

The bark covers and stiffens the spines on many woody trees and shrubs.

Bark =

epidermis + periderm + cortex + phloem + vascular cambium

Wood =

secondary xylem only!

Pith =

a small percentage

of tree diameter

at maturity