THE STEM

STRUCTURE & FUNCTIONS

General Terminology

• Annual – a plant that completes its life cycle (seed, flower, fruit and seed) in one growing season.

• Ex. Crop plants

• Biennial – a plant that requires two growing seasons (2 years) to complete its life cycle.

• Ex. Asparagus

– First year is vegetative growth (non-reproductive part)– Second years growth is reproductive – flowers and

fruits

General Terminology

• Perennial – longer lived vegetative plants, continue to live after flowering and fruiting reproduce for many years (seasons).– Examples

• Trees, shrubs, many flowering plants: impatiens, periwinkle, dianthus and house plants.

• Stem – the part of the axis of vascular plants that is above ground, as well as anatomically similar portions below ground – rhizomes and corms.

FUNCTIONS

• Transport of water & nutrients to the top

• Support

• Storage

• Photosynthesis (sometimes)

Development• Primary growth

– From the shot apical meristemprimary tissues & increased stem length, origin of the leaves

• Secondary growth (mostly in woody plants; it is limited in herbaceous plants

– From the vascular cambium secondary tissues & increased diameter

– From the cork cambium periderm

• Apical meristem at the tip– From apical meristem you have development

of the leaf primordia. As the shoot continues to grow the bud primordia develop in the leaf axil. (These will develop into other shoots and leaves)

– As the leaf and bud are formed a strand of xylem and phloem known as the leaf trace, branches off the main vascular cylinder and goes into the stem.

Development

TISSUE PATTERNS

• Stele – a central cylinder of vascular tissue (inside of the cortex)

• Several different arrangements

TISSUE PATTERNS

– Protostele – Most primitive form, consisting of a solid core of conducting tissues, with the phloem surrounding the xylem. Found in many of the extinct vascular plants as well as in many of the living seedless plants.

phloem

xylem

TISSUE PATTERNS

• Siphonostele – more derived condition, tubular arrangement with pith in the center surrounded by the vascular tissue. Found in most ferns and many gynosperms and angiosprems.

XylemPith

Phloem

TISSUE PATTERNS

• Eustele – Primary vascular tissues are arranged in different vascular bundles around the pith. Vascular bundles are strands of tissue comprised of xylem and phloem and procambium.

Vascular Bundle

Monocot vs. Dicot

• Monocot – Plants that produce one cotyledon, or seed leaf. (Ex. grasses, wheat, palms, barley)

• Dicot – Plants that produce two cotyledon.

• Cotyledon – “seed leaf”, the first leaf formed in a seed.

Monocots

• Outer layer of epidermis, with an underlying layer of dead sclerenchyma tissues (fibers)

• Vascular bundles scattered throughout the stem, with a higher concentration area the outer perimeter. Which also serves to strengthen the stem.

• The remaining portions of the stem is composed of parenchyma tissues, or ground tissues.

Cross Section of A Monocot Stem

Stem x.s. of Asparagus

Cross Section of A Monocot StemStem X. S. of Zea mays (corn)

Vascular BundleAir Space

Seive tube elements and companion cells

Center

Outside

Vessels and tracheids

Xylem

Phloem

Vessels

Dicots

• Outer epidermis, with the underlying ground tissues (collenchyma, parenchyma and sclerenchyma) divided into a cortex and pith.

• Vascular bundles arranged in a ring underneath the cortex of the stem. With ground tissues between the bundles that are continuous between the pith and cortex – interfascicular regions.

Cross Section of A Dicot Stem

Interfascicular regions

Pith

Cortex

Stem X.S. of Phaseolus vulgaris

Activity of the Vascular Cambium Results in Addition of Secondary Tissues

Secondary Growth of The Stem

As in roots, the activity of the vascular cambium in the stem gives rise to xylem towards the center and phloem towards outside. As new cells are added the diameter of the stem increases and the cortex and the epidermis are crushed and replaced.

The Vascular System of A PlantFrom The Root to The Stem

Stem

Root

Secondary Growth of The Stem

• Formation of secondary vascular tissue– Secondary xylem & phloem

• Periderm

Development

• Secondary growth (mostly in woody plants; it is limited in herbaceous plants

– From the vascular cambium secondary tissues (xylem and phloem & increased diameter

– From the cork cambium periderm

Growth and Wood types

Growth rings – growth layers which correspond to growing seasons.

Early wood – “spring wood”, abundance of rain and cooler temperature which allow cells to grow larger and more rapid.

Late wood – “summer wood”, less than optimal growing conditions, growth slows.

Sapwood – wood found between vascular cambium and heartwood, functions in the transport of water and solutes.

Heartwood – wood found in the center of the tree trunk, usually darker because of resins, oils and gums. Doesn’t transport water or solutes.

Stem Structures

• Apical or terminal bud (tip), extends the length of the stem.

• Lateral or axillary buds – develop the shoots or branches and flowers from the side.

• Bud scale – terminal bud produces terminal bud scales which protect buds from temperatures, desiccation and pathogens.

• Leaf scars – scar left on the twig when a leaf falls off.• Axil – upper angle between a twig or leaf and the stem

from which it grows.• Axillary Bud – Bud that grows from the axil.

Stem Modifications

• Rhizomes – horizontal, below ground stems, often near the surface with adventitious roots. (Irises, and grasses)

• Tuber – fleshy underground stem, typically used for the storage of starches.

• Bulb – large buds with small conical stems surrounded by large fleshy leaves.

• Corms – Similar to bulbs, but differ in that they are composed mainly of stem materials.

Stem Modifications