Topic 9: Plant Growth - Shoots

Primordia

• These are organ precursors that form on the margins of the shoot apical meristem (SAM)

• Vegetative meristems specifically produce leaf primordia

Simple leaf

• A type of leaf that possesses a single, undivided blade

• common on monocots

• veins usually parralel

Compound leaf

• A leaf in which the blade is divided into multiple leaflets

• common in eudicots

Palisade mesophyll

Chloroplast-rich, elongated cells located on the upper side of the leaf that are specialized for capturing light

Spongy mesophyll

A porous layer of cells with a high surface area to volume ratio that facilitates gas circulation within the leaf

Bundle sheath cells

Cells that surround the veins and regulate the transfer of substances between the mesophyll and the vascular tissue

Rhizomes

Stems that grow underground and have the ability to put up new shoots

Stolons

Also known as "runners," these are stems that extend along the soil surface, establishing new plantlets at their nodes

Root tubers

While the worksheet uses this term, the sources describe stem tubers as the enlarged ends of rhizomes or stolons used for food storage

Spines

These are modified leaves

Thorns

These are modified axillary branches

Prickles

These are modified epidermal extensions of the stem; unlike thorns, they have no vascularization

Secondary growth

This process drives the thickening of stems and roots, primarily in eudicots and conifers, through the action of lateral meristems

• doesnt happen in monocots

Vascular cambium

A cylinder of lateral meristematic tissue that runs between the xylem and phloem, adding more vascular tissue to widen the shoot

Cork cambium

Lateral meristematic tissue that develops in the cortex and produces cork

Cork

A tough, waxy covering produced by the cork cambium that protects the stem from water loss and biological invaders.

Secondary xylem

Xylem cells formed to the inside of the vascular cambium; these cells eventually become wood

Secondary phloem

Phloem cells formed to the outside of the vascular cambium

Suberin

A waxy substance produced by cork cells that waterproofs the stem and prevents leakage

Bark

This includes all tissue external to the vascular cambium, specifically the secondary phloem, cork cambium, and cork

Annual rings

These develop from the uneven seasonal growth of secondary xylem

• smaller rings in the winter/fall

• larger rings in spring

Heartwood

The older layers of secondary xylem that have ceased to conduct water

Sapwood

The most recent layers of secondary xylem that are still active in water conduction