Bio 402 exam 2 Xylem Slides

What are the three primary functions of xylem?

Water and mineral conduction, mechanical support, and storage of substances like carbohydrates or defensive compounds.

From what meristems do primary and secondary xylem originate?

Primary xylem from the procambium, secondary xylem from the vascular cambium.

What is the main difference between primary and secondary xylem in function?

Primary xylem supports elongation and early conduction; secondary xylem provides long-term transport and mechanical strength (wood).

Which two broad systems make up secondary xylem structure?

The axial system (vertical conduction/support) and the radial system (rays for lateral transport and storage).

What are the two main types of tracheary elements?

Tracheids and vessel elements.

How do tracheids differ from vessel elements structurally and functionally?

Tracheids are long, tapering, and conduct through bordered pits; vessels are shorter, wider, and joined by perforation plates for faster conduction.

What is the evolutionary trend in vessel perforation plates?

From scalariform → reticulate → simple, showing a shift toward greater efficiency.

What are tracheary elements made of, and are they living at maturity?

Lignified secondary walls; dead at maturity.

What is the major conduction advantage of vessels over tracheids?

Vessels provide continuous water columns, enabling faster flow but greater embolism risk.

In which plant groups are vessels absent or rare?

Gymnosperms (except Gnetales) — they rely solely on tracheids.

What are fiber-tracheids?

Transitional cells between tracheids and fibers; retain vestigial bordered pits but mainly support.

How do libriform fibers differ from fiber-tracheids?

Libriform fibers lack functional pits, are more lignified, and provide only mechanical support.

Are xylem fibers living or dead at maturity?

Dead.

Are xylem parenchyma cells living or dead?

Living at maturity.

What are the main functions of xylem parenchyma?

Storage, lateral transport, formation of tyloses, and secretion of extractives in heartwood.

What are tyloses and why are they important?

Balloon-like outgrowths from parenchyma into vessel lumens that block damaged or aging vessels, preventing pathogen spread.

What are the two orientations of xylem parenchyma?

Axial parenchyma (vertical) and ray parenchyma (horizontal).

Axial parenchyma (vertical) and ray parenchyma (horizontal).

Thin areas of the secondary wall that allow water passage between adjacent cells.

What is the difference between simple and bordered pits?

Simple pits lack secondary wall overarches; bordered pits have secondary wall rims that form a pit chamber and aperture.

What is the torus–margo structure, and what is its function?

A bordered pit type in gymnosperms; torus acts as a valve that seals off embolized cells; margo allows water flow.

What causes cavitation and embolism in xylem?

Formation of air bubbles from drought stress, freezing, or mechanical damage.

How do plants prevent or recover from embolisms?

Torus–margo pits, tyloses, and sometimes nighttime refilling.

What is protoxylem?

The first-formed xylem with annular or helical secondary walls that allow stretching during growth.

What is metaxylem?

Later-formed xylem with pitted or reticulate walls for rigidity and higher conductance after elongation stops.

Why must protoxylem be extensible?

To accommodate elongation of the growing organ.

What is the developmental order of xylem formation?

Protoxylem → Metaxylem → Secondary Xylem

What produces secondary xylem?

The vascular cambium, a lateral meristem.

What are growth rings, and how do they form?

Alternating bands of earlywood (large, thin-walled cells) and latewood (small, thick-walled cells) caused by seasonal cambial activity.

What is the difference between heartwood and sapwood?

inner, dark, non-conducting, extractive-rich.
Sapwood: outer, lighter, conducting, living.

What is reaction wood and why does it form?

Abnormal wood that develops to correct stem position.

• Compression wood (conifers): lower side, lignin-rich.

• Tension wood (angiosperms): upper side, cellulose-rich.

What are the four major stages of tracheary element differentiation?

Cell elongation → Secondary wall deposition → Lignification → Programmed cell death (PCD).

Which hormones initiate xylem differentiation?

Auxin and Cytokinin

Which hormones promote secondary wall deposition and cell death?

Gibberellin and Ethylene.

Which hormone generally inhibits xylem differentiation?

Abscisic acid (ABA).

What experimental system demonstrated tracheary element transdifferentiation?

Zinnia elegans mesophyll cells forming tracheary elements in vitro with hormonal cues.

What evolutionary trends are seen in tracheary elements?

Shorter elements, simpler perforation plates, thinner pit membranes, and separation of conduction vs support roles.

What is the evolutionary advantage of vessel elements?

More efficient water transport, allowing larger body size and faster growth.

Why are conifers more resistant to drought-induced embolism?

Their tracheid-based system with torus–margo pits provides hydraulic safety.

What is the tradeoff between hydraulic safety and efficiency?

Wider vessels = greater efficiency but higher embolism risk; narrow tracheids = lower efficiency but greater safety.

How does xylem anatomy reflect plant environment?

Dry-adapted species have narrow vessels and thicker walls; wet-adapted species have wider vessels and thinner walls.