The stem: Definition and Functions

Stem

It is an axis.

Functions of stem

Support aerial parts, transports water and solutes between roots and leaves, produce carbohydrates, photosynthesizes, asexual reproduction, and store materials.

Importance for the stem to elevate the aerial parts of the plant.

This helps in reaching sunlight for the leaves; maximizes photosynthesis and makes it accessible to animals that help in pollination/reproduction.

Evolution of plants

The xylem and phloem helped in long distance transport.

Asexual reproduction

Stem cutting, marcotting, and grafting.

Stem cutting

A piece of stem is buried in the soil, including at least one leaf node

Marcotting

A.k.a. air layering; induces roots above ground

Grafting

Joining two plants together.

Water and starch

Stored materials in stems

Above-ground modified stem

Runners/stolons, cladodes, succulent stems, thorns

Runners/stolons

A slender stem that grows horizontally along the ground that gives rise to roots and aerial branches at nodes. Have long and thin internodes, and leaves that don't expand.

Cladodes

Flattened stems that perform photosynthesis.

Cladophylls or phylloclades

Cladodes

Function of cladodes

Reduces water loss, stores water, and photosynthesize.

Succulent stem

Stems with fleshy tissues that store water.

Thorns

These are specialized sharp, pointed stems that can be recognized as it is subtended by a leaf.

Thorns vs prickles

A thorn is actually a specialized stem, while a prickle is just an outgrowth of the stem.

Underground modified stem

Bulb, rhizome, corm, and tuber

Bulb

Short roots with thick, fleshy leaves; the stem is reduced to a small knob.

Rhizome

Fleshy horizontal stems that allow a plant to spread underground.

Corm

Vertical, thick stems that have thin, papery leaves

Tubers

Stems used for storage

Node

This is where the leaves of the plant is attached

Phyllotaxy

The way leaves are arranged around the stem

Spiral

Like a helix; leaves not aligned with nearest neighbors (arranged in a Fibonacci spiral) 137.5 degree (golden ratio)

Distichous

Arranged in two rows and can be alternate

Opposite

Two leaves per node

Decussate

Opposite leaves arranged 90 degrees relative to each other.

Whorled

Three or more leaves per node.

Intercalary meristem

Allows grasses to grow fast - rapid elongation in the stems of grasses.

Internode

The space/region between nodes.

Plants that has very short internodes and are packed with leaves

Rosette plants

Leaf axil

Stem area just above the point where the leaf attaches.

Axillary/lateral bud

A miniature shoot with a dormant apical meristem and several young leaves. It is either vegetative or floral.

Apical dominance

Shoot apex inhibits the growth of lateral buds so that the plant may grow vertically.

Terminal bud

At the extreme tip of the stem.

Leaf scars

Left behind when leaves are shed.

Lenticles

Slits in the stem that facilitates in gas exchange in the plants.

Bud scales

Protects the axillary/lateral bud from harsh conditions.

Pseudostem

Fake trunk that is made of leaf sheaths.

Epidermis

Protective covering over the surface.

Made of single layer of parenchyma cells that secretes cutin to make the wall impermeable to water (prevent water loss)

Epidermis

Cuticle

Formed that is built by epidermis.

Epidermis cell types

Guard cells and trichomes

Guard cells

Gateways for CO2 to enter the impermeable epidermis.

Trichomes

The hairs that make it difficult for animals to land on, walk on, or chew into a leaf; also provide shade, and create a layer of immobile air next to a leaf surface.

Cortex

Composed of mostly large thin-walled parenchyma cells that shows little to no structural deifferentiation.

Cortex main functions

Provide support and perform metabolic processes.

Cortex contained/stored this

Carbohydrates or other substances such as resins, latex, essential oils, and tannins.

Pith

Ground tissue that is internal to the vascular tissue, A.k.a medulla. Soft, spongy parenchyma cells, which in some cases can store starch.

Vascular bundles

Occurrence of xylem and phloem tissues together that is interior to the cortex.

Dicot vascular bundles

Arranged in a ring around the pith

Monocot vascular bundles

Scattered

Monocot stem

Looks like a smiley face.

Vascular cambium

Contributes to secondary growth; it produces the secondary xylem and phloem.

Fascicular and interfascicular cambium

Can only be differentiated if the plant is still young.

Fascicular cambium

Differentiate from the latent procambium

Interfascicular cambium

From the cortical cells, separating the vascular cambium.

Secondary growth

Does not undergone by monocots, so the monocots doe not have fascicular and interfascicular cambium.

Soft wood

Comes from gymnosperms that has few or no fibers and have softer consistency. It also lacks vessels such gymnosperms.

Hardwood

Usually developed from angiosperms that has large amounts of fibers, making them strong, tough, and good, for construction and have vessels.

Growth ring

Ring patters

Dendrochronology

Study of growth rings.

Thick ring

Warm or wet year

Thin ring

Cold or dry year

Formation of the growth ring/annual ring

Spring wood and summer wood

Composition of secondary xylem

Softwood and hardwood

Spring wood

Lighter, early wood, and most wood laid down in spring, where greatest growth occurs.

Summer wood

Darker, late wood, and growth is slower and xylem cells are smaller

Scion and rootstock

In grafting

Scion

What to attached in grafting

Rootstock

Where to attached in grafting

Faster growth and cloning of the scion

Purpose of grafting

Secondary growth

Formed from vascular cambium that transport nutrients.

The external portions

It gets are crushed and die when the secondary phloem grows

Periderm

Made up of cork cambium + layer of cork cells + phelloderm

Function of periderm

Offers temporary protection

Periderm tissue composition

Phellogen, phellem, phelloderm

Phellogen

Cork cambium which leads to the production of periderm

Phellem

Outer derivative of phellogen: cells in the cork cambium undergo cell division.

Phelloderm

A cell or two that matures into layer of parenchyma.

Phellem

Cork cambium

Phellogen

Cork cambium

Phelloderm

Parenchyma cells, internal to phellogen

Wood

Xylem

Bark

Secondary phloem + cork

Differentiation of fascicular cambium in roots and stem

Procambium

Differentiation of interfascicular cambium in roots

Pericycle

Differentiation of interfascicular cambium in stem

Cortical cells

Ray initials

Short, cuboidal, and when undergoing cell division, forms two daughter cells.

Function of ray initials

It helps to move substances, and store water, carbohydrates.

Fusiform initials

Long, tapered cells, have thin cell walls, and when they divide, cells that are interior to the cambium become secondary xylem (vessels, tracheids, fibers; forms the wood).

Cambial initial

A one cell that always remain when ray and fusiform initials undergo division.

Wood

It never forms to the exterior of the vascular cambium

Bark

Never forms on the interior side.

Heart wood

Located in the middle that is darker, drier (due to tannins too), more fragrant, dead, and does not function for water conduction.

Tylosis

The mechanism that seals off fungal and bacterial growth.

Heartwood

The parenchyma cells undergo metabolic changes, and produce lignins, phenolic compounds, and other dark-colored aromatic substances.

Sapwood

Located in the exterior portion that is lighter, moister wood, and the living portion of the stem.