Bio 402 exam 2 xylem Book

What are the two main categories of xylem, and from what meristems do they originate?

Primary xylem arises from the procambium during primary growth; secondary xylem (wood) arises from the vascular cambium during secondary growth.

How does xylem illustrate the integration of transport and mechanical functions in higher plants?

Its tracheary elements ensure water conduction, fibers provide mechanical strength, and parenchyma cells enable storage and repair—demonstrating a multifunctional tissue that unites transport and support within the plant axis.

Contrast the structural and functional differences between tracheids and vessel elements.

Tracheids are elongated, tapering, and communicate laterally via bordered pits; they conduct and support. Vessel elements are shorter, wider, and joined end-to-end by perforation plates to form continuous vessels, allowing more efficient axial flow but higher embolism risk.

What is the evolutionary trend observed in vessel element structure across angiosperms?

Progressive shortening of elements and simplification of perforation plates (from scalariform → simple) reflecting increasing hydraulic efficiency and evolutionary advancement.

Explain the role of pits in tracheary elements and how bordered pits minimize embolism.

Pits allow lateral water movement; bordered pits possess a torus–margo structure that can seal off an air-filled conduit, maintaining hydraulic continuity.

Differentiate between fiber-tracheids and libriform fibers.

Fiber-tracheids retain vestigial bordered pits and can conduct minimally; libriform fibers are highly lignified, lack functional pits, and specialize solely in mechanical support.

Describe the structure and function of xylem parenchyma.

Living cells that store starch and phenolics, participate in lateral transport via rays, and can form tyloses (balloon-like intrusions) into vessels to isolate damaged or aged conduits.

What are tyloses, and what is their functional significance?

Outgrowths from parenchyma that block vessel lumens, preventing spread of pathogens and marking the transition of sapwood to non-conducting heartwood.

Outline the sequential stages of tracheary element differentiation

(1) Cell elongation → (2) Secondary wall deposition → (3) Lignification → (4) Programmed cell death (loss of protoplast, functionalization of lumen).

Which plant hormones coordinate tracheary element differentiation, and what are their roles?

Auxin and cytokinin initiate xylem formation; gibberellin and ethylene promote secondary wall development and PCD; abscisic acid can modulate or inhibit differentiation

How do Zinnia elegans mesophyll cells contribute to our understanding of xylem development?

They can transdifferentiate into tracheary elements in vitro when exposed to hormonal cues, proving that xylem identity can be induced epigenetically from non-vascular cells.

How do protoxylem and metaxylem differ in structure and function?

Protoxylem forms first with annular/helical secondary walls permitting stretch during elongation; metaxylem forms later with pitted or reticulate walls for greater rigidity and conductance after elongation stops.

What developmental transition defines secondary growth, and what tissue drives it?

The onset of radial thickening via the vascular cambium, which produces secondary xylem internally and secondary phloem externally

Describe the axial and radial systems of secondary xylem.

The axial system (vertical) includes tracheary elements, fibers, and axial parenchyma; the radial system (horizontal) consists of rays that store and laterally distribute solutes.

What causes growth rings, and what information do they provide?

Seasonal alternation of cambial activity produces earlywood (large lumens, thin walls) and latewood (small lumens, thick walls); their pattern records annual climatic conditions and growth rate.

Define reaction wood and distinguish its two types.

Abnormal wood formed in leaning stems or branches: compression wood (gymnosperms, lower side, lignin-rich) and tension wood (angiosperms, upper side, cellulose-rich gelatinous layer).

How does heartwood differ chemically and functionally from sapwood?

Heartwood is non-conductive, impregnated with tannins, resins, and extractives, enhancing decay resistance; sapwood remains alive, active in water conduction and storage.

Contrast gymnosperm and angiosperm wood composition.

Gymnosperms: nearly all tracheids, simple rays, resin ducts.
Angiosperms: mix of vessels, fibers, parenchyma, and complex rays; typically ring-porous or diffuse-porous based on vessel distribution.

What anatomical features are key for wood identification?

Vessel arrangement (porosity pattern), type of perforation plates, ray height and width, axial parenchyma pattern, and presence of resin ducts or tyloses.

Explain how cambial organization affects wood texture and pattern.

Storied cambia produce tiered cell files yielding fine-grained wood; nonstoried cambia yield irregular grain typical of coarser woods.

How do fusiform and ray initials contribute differently to secondary xylem?

Fusiform initials divide periclinally to form axial elements (vessels, fibers, axial parenchyma); ray initials produce horizontal ray parenchyma for radial transport and storage.

What environmental factors control the periodicity of cambial activity?

Temperature, photoperiod, and water availability regulate dormancy and reactivation, causing annual ring formation in temperate species.

Describe how reaction wood formation exemplifies plant mechanical adaptation.

It re-establishes vertical posture by altering cell wall composition and growth stress, demonstrating plastic developmental response to mechanical strain.

How does xylem evolution mirror the terrestrialization of plants?

The progression from tracheids → vessel elements parallels the move toward greater hydraulic efficiency, allowing taller growth and enhanced carbon gain on land.

Why is secondary xylem considered both a developmental and ecological archive?

It records cellular differentiation patterns and environmental history (climate, stress, injury) in its annual rings and anatomical features.