Stems-II

Primary thickening meristem occurs in many monocots

Primary thickening meristems

Primary Thickening Meristem

Banana “tree”

Foxglove vascular ring

Eustele in dicots

Alfalfa and ragweed VB

Clover fasicular cambium spreads to interfasicular region

Secondary growth restricted to VB

Ordinary secondary growth in dicots and gymnosperms secondary xylem and phloem develops from the vascular cambium

Anomalous Secondary Growth

• Localized cambial activity because some regions of cambium die

• Uneven amounts of xylem and phloem

• Multiple cambia develop

• Concentric (or successive) layers of vascular cambium

• Monocots

MonocotsYucca brevifolia Cocus nutifera

Cordyline (Ti plant)

Secondary Growth in Monocots

• Meristem (cambium) becomes distinct outside the outmost VB in the stem

• Secondary Thickening Meristem produces – Cells outward that differentiate into

parenchyma becoming part of a secondary cortex

– Cells produced inward are either • Conjunctive parenchyma that become sclerified• Parenchyma that differentiates into fibrovascular

bundles (called secondary bundles)

Secondary thickening meristem in Cordyline (Ti plant)

Palm trunk Royal palm

Palm trunk

Stems (gymnosperms, dicots, and monocots) also contain a variety secretory structures

Resin ducts in Pinus

and in Artemesia

Ducts may contain resins, gums, mucilage produced by parenchyma (epithelial) cells lining the duct

Resin duct in poison ivy stem

Oil and mucilage cells

Laticifers

• Secretory structures (of one or more cells) that produce latex (latex is a milky secretion produced by many plants with varied composition– Non-articulated laticifers consist of extremely

elongated single cells (cell growth keeps pace with the growth of the stem)

– Articulated laticifers are composed of many individual cells that are usually interconnected by holes in their common walls (like vessels)

Non-articulated laticifer in Euphorbia

Articulated laticifers