Tree Biology Practice Questions

Correct answer: C. Inner bark (phloem)

The inner bark is the phloem, the tissue that moves sugars from the leaves to the roots and to other growing parts of the tree. Sapwood is the tempting wrong answer because it also conducts, but it carries water and dissolved minerals upward from the roots. Heartwood no longer conducts, and the outer bark protects the tree rather than transporting food.

• A: Sapwood conducts water and minerals up from the roots, not sugars from the leaves.
• B: Heartwood is the non-conducting core of the trunk.
• D: Outer bark protects the inner tissues and limits water loss; it does not move sugars.

Source: Wood Handbook: Wood as an Engineering Material (FPL-GTR-190), Chapter 3: Structure and Function of Wood, USDA Forest Service, Forest Products Laboratory

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Correct answer: C. Fungi in a mutually beneficial partnership with roots

Mycorrhizae are symbiotic, mutually beneficial associations between certain fungi and plant roots, and the partnership makes the plant better at absorbing water and nutrients. Nitrogen-fixing nodules are the closest distractor because they are also a beneficial root partnership, but they involve bacteria rather than fungi. Root rots and water-blocking mats describe harmful organisms.

• A: This describes a harmful root-rot fungus, not a beneficial association.
• B: Nodule bacteria are a different partnership; mycorrhizae involve fungi.
• D: Mycorrhizae improve water uptake rather than blocking it.

Source: Vegetative Plant Parts (EM 9904), Oregon State University Extension Service

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Correct answer: A. Roots in saturated soil could not get enough oxygen

Roots need oxygen for respiration, the process that releases energy from sugars. When soil stays saturated, water fills the pore spaces, oxygen runs short, and roots stop growing and may die. Wet soil does not stop transpiration from the leaves or dilute phloem sugars, and constant moisture does not trigger root dormancy.

• B: Soil water does not enter the phloem and dilute its sugars.
• C: Transpiration happens at the leaves and is not stopped by wet soil.
• D: Root dormancy is not triggered by constant soil moisture.

Sources: Vegetative Plant Parts (EM 9904), Oregon State University Extension Service ; Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

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Correct answer: D. It releases energy from sugars, day and night

Respiration is the controlled breakdown of sugars to release energy for growth and maintenance. Unlike photosynthesis it does not need light, so it continues both day and night, and it takes place in all living cells, including roots and stem tissues. Building sugars from carbon dioxide and water describes photosynthesis, the usual mix-up.

• A: Respiration occurs in all living cells, in darkness as well as light.
• B: Building sugars from carbon dioxide and water is photosynthesis.
• C: Living cells keep respiring during dormancy, drawing on stored reserves.

Source: Plant Growth and Development (EM 9901), Oregon State University Extension Service

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Correct answer: A. Wall 1, the plugged vertical conducting cells

Wall 1 forms when the vessels or tracheids above and below a wound plug, completing the tops and bottoms of the compartments. It is the weakest of the four walls, which is why columns of discolored and decayed wood usually extend farthest vertically. Wall 2 resists inward spread, Wall 3 (the rays) resists spread around the trunk, and Wall 4, the barrier zone, is the strongest.

• B: Wall 2 resists inward spread toward the pith and is the second-weakest wall.
• C: Wall 3, the rays, resists lateral spread around the trunk and is fairly strong.
• D: Wall 4 is the strongest wall and separates old wood from wood formed after wounding.

Source: Compartmentalization of Decay in Trees (Agriculture Information Bulletin 405), USDA Forest Service

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Correct answer: B. Auxin from the tip no longer suppresses the lateral buds

Auxin produced in the stem tip suppresses the growth of lateral buds, a pattern called apical dominance. Removing the terminal bud removes that auxin source, so buds below the cut are released and grow. Ethylene is a gaseous hormone associated with aging and ripening, not the release of lateral buds, and small changes in water or light do not explain this hormonal response.

• A: Water supply is not what holds the buds back; auxin from the tip is.
• C: Ethylene is linked to aging and ripening, not to releasing lateral buds.
• D: Light does not explain it; removing the auxin source releases the buds.

Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

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Correct answer: B. Grafting or rooting cuttings from the tree

Asexual (vegetative) propagation, such as cuttings, layering, budding or grafting, reproduces the parent's genetics in the new plant (in a grafted tree, the top grown from the scion; the rootstock is usually a different seedling), which is why named cultivars are propagated this way. Seeds and the seedlings growing under the tree come from sexual reproduction, which combines genetic material from two parents, so the offspring differ from the cultivar. Cross-pollination likewise produces new, variable offspring.

• A: Seeds come from sexual reproduction, so seedlings are not identical to the parent.
• C: Volunteer seedlings are products of sexual reproduction and vary genetically.
• D: Cross-pollination creates genetically new offspring, not copies.

Source: Extension Gardener Handbook, Chapter 13: Propagation, NC State Extension

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Correct answer: C. Layering

In layering, a stem develops roots while it remains attached to the parent plant; the rooted stem is cut free afterward. Because it stays connected, water stress is low and food and nutrient supplies remain high while roots form. Grafting and budding join a scion to a rootstock, and division splits a plant that already has its own roots.

• A: Grafting joins a scion to a rootstock; it does not root an attached stem.
• B: Budding inserts a single bud onto a rootstock.
• D: Division splits a plant that already has its own roots.

Source: Extension Gardener Handbook, Chapter 13: Propagation, NC State Extension

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Correct answer: B. Carotenoids present all season show as chlorophyll fades

Carotenoids, the yellow and orange pigments, are present in leaves all season but are masked by the much larger amount of green chlorophyll. In autumn, chlorophyll production stops and the existing chlorophyll breaks down, unmasking the yellows. Anthocyanins are produced in autumn from trapped sugars, but they give reds and purples, not yellows, and freezing temperatures reduce rather than create fall color.

• A: Freezing temperatures injure leaves and reduce autumn color rather than creating it.
• C: Anthocyanins made from trapped sugars produce reds and purples, not yellows.
• D: Tannins are already in the leaves and give browns, not the yellow color.

Source: Why Leaves Change Color: The Physiological Basis (FNR-FAQ-5), Purdue University Cooperative Extension Service

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Correct answer: D. Normal seasonal shedding of the oldest needles

Evergreen conifers keep their needles for more than one season and shed the oldest, innermost needles each autumn after they turn yellow or brown. Even yellowing of the interior needles while the current-year needles at the tips stay green is the typical pattern of this natural process. Homeowners often mistake it for a disease or insect problem, but on an otherwise vigorous tree in fall no treatment is needed.

• A: Root disease would not produce this even, seasonal pattern on a vigorous tree.
• B: This uniform inner-crown yellowing in autumn is natural, not a needle disease.
• C: Drought injury is not the usual cause of uniform autumn yellowing of older needles.

Source: Why Leaves Change Color: The Physiological Basis (FNR-FAQ-5), Purdue University Cooperative Extension Service

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