Stem Lecture

Functions

Support of Leaves, Flowers, Fruits Conduction of Water, Minerals, Sugars, etc.

Photosynthesis Storage Defense

Stems support a display of leaves.

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

Asclepias

- milkweed

The stem supports a display of flowers

Cercis canadensis

- redbud

The stem supports a display of fruits.

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

Ipomoea nil

- morning glory

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

Structure to Provide Functions

Support of Leaves, Flowers, Fruits Conduction of Water, Minerals, Sugars, etc.

Photosynthesis Storage Defense

Typical Stem Cross Section Epidermis Cortex A ring of vascular bundles

Helianthus annuus

sun flower annual Pith

Epidermis - window, reduce water loss Cortex Collenchyma - extensible support Cortex Parenchyma - photosynthesis, etc.

Phloem Fibers - rigid support Functional Phloem - conduct sugars etc. away Xylem from leaf to rest of plant Vascular Cambium - adds 2° xylem and 2° phloem -conduct water and minerals up from soil Pith -water storage, defense?

VIP Stem:

Provide both

name

and

function

labels: Epidermis: reduce evaporation, gas exchange Cortex: photosynthesis, collenchyma support Vascular Bundles: conduction Pith: water storage? defense? disintegrate?

outside to center

Vascular Bundle:

Phloem Fibers: support Functional Phloem: conduct CH 2 O away from leaf Vascular Cambium: add 2° Xylem and 2° Phloem Xylem: conduct minerals up from soil

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

- grape

The vascular cambium makes 2° tissues:

Vitis vinifera

- grape

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).

spring of the

next

winter of that year year fall of that year mid-summer of one year

Three years of Secondary Growth

Tilia

- basswood

Secondary Phloem Secondary Xylem

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

cutin suberin

Cork Cells Cork Cambium Phelloderm

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!

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

Sequoia sempervirens

- giant sequoia

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