Exercises: Plant tissues

Name the main plant tissues (meristems, parenchyma, supporting tissue, vascular tissue, and dermal tissue) and summarise the function of each in one sentence.

Where are meristems located in a plant? Explain why, thanks to them, a plant can grow throughout its life, unlike an animal.

Distinguish between parenchymal tissue and supporting tissue: cellular appearance, cell wall thickness, and role in the plant, indicating where each is abundant.

Explain what a stoma is, which cells form it and which two vital processes its opening and closing regulate (gas exchange and water loss).

Distinguish between epidermis and cuticle, and explain how together they protect the leaf against desiccation and against the entry of pathogens.

What is chlorenchyma or chlorophyllous parenchyma? Distinguish between palisade mesophyll and spongy mesophyll and relate them to the use of light and to gas exchange.

Explain why the conducting cells of the xylem (tracheids and vessel elements) are dead and hollow at maturity, and why this, far from being a problem, improves their function.

What is the cambium and where is it located in a woody stem? Indicate which tissue it produces towards the inside and which towards the outside, and what happens to a tree if a complete ring of bark is removed from it.

Look at the leaf cross-section in the image. a) Identify the numbered structures and the tissue to which each belongs. b) Explain how each tissue relates to photosynthesis or sap transport. c) State through which structure air enters and exits, and how this exchange is regulated.

Xylem and phloem are the plant's vascular tissues. Compare their conducting cells (tracheids and vessel elements versus sieve tubes), the direction of transport, and the sap moved by each tissue.

Collenchyma and sclerenchyma support the plant without lignifying its entire surface. Explain the difference between these two supporting tissues and relate it to the flexible habit of an herbaceous plant versus the rigidity of a tree.

Explain how an annual growth ring forms in the trunk (early wood and late wood), what dendrochronology is, and what is deduced from very narrow or very wide rings about the climate of each year.

Cork is obtained from the cork oak without felling it. Explain what the phellogen (cork cambium) is, what cork is and why the bark can be stripped periodically, as well as the ecological and economic value of the cork oak woodland.

Describe the zones of a young root (root cap, zone of division, zone of elongation and zone of absorbent hairs) and explain which tissues take part in the absorption of water and how the root hairs multiply the surface.

Compare the leaf of a pine (a needle), that of a holm oak and that of a deciduous oak: cuticle, shape, stomata and leaf lifespan. Relate each trait to the climate it is adapted to (cold and summer drought versus mild winters and humid summers).

The vascular bundles connect root, stem and leaf. Explain how they are arranged in the stem and in the vein of the leaf, how you would tell xylem and phloem apart in a section under the microscope (size and appearance of the cells, position) and what each one transports.

During a botanical expedition, leaf cross-sections are obtained from two unknown plant species from extreme environments:

• Species A: multi-layered epidermis with an 18 µm cuticle, stomata confined exclusively to stomatal crypts with abundant trichomes on the abaxial surface, and a very compact palisade mesophyll.

• Species B: single-layered epidermis with an imperceptible cuticle, stomata located only on the adaxial surface, and a mesophyll made up almost entirely of large, septate intercellular cavities.

1. a) Deduce with justification the ecological characteristics of the habitat (water availability, radiation exposure, and the medium in which they float or grow) for each of the two species based on their histological features.
2. b) Predict what immediate physiological consequences Species B would suffer if a summer drought drastically reduces the water level and its leaves are left directly exposed to dry, warm air.

The trunk of a felled pine tree in a Mediterranean area is analyzed, showing that in the rings corresponding to the years 2012 and 2014 the total wood thickness is less than 1.2 mm, with scarce earlywood, whereas in 2010 it exceeded 4.5 mm. Furthermore, a deep wound inflicted in 2008 on the outer bark was completely sealed by a whitish protective tissue.

1. a) Explain at the cellular level which secondary meristem is responsible for the formation of tree rings and how water availability in spring determines the size of earlywood cells compared to latewood cells.
2. b) Identify the secondary meristem involved in healing the superficial wound by producing cork (suber) and reason what would have happened to the tree's survival if the wound had deeply severed a continuous ring of phloem all around the trunk.
3. c) A student claims: "If we hammer a nail 1.5 meters high into the trunk of a young tree, ten years from now the nail will be higher than 3 meters because the tree grows upward." Refute their claim based on the function and location of primary and secondary meristems.

A team of students subjects several bean seedlings to two treatments for three weeks: batch A with daily watering and batch B without watering. When observing leaf sections under the microscope, a student writes in their notebook: "The leaves of batch B defended themselves by synthesising a cuticle three times thicker in a few days and eliminating half of their stomata so as not to lose water."

1. a) Correct the student's conceptual errors by distinguishing between short-term physiological responses and evolutionary or developmental anatomical adaptations.
2. b) Describe what real histological and cytological differences you would expect to observe under the microscope between both batches at the level of the mesophyll (cell turgor, vacuolar volume, and intercellular spaces) and the state of the stomatal guard cells.
3. c) Justify how the state of the guard cells in batch B will affect the photosynthetic rate and the transport of xylem sap from the root through the xylem.

Crassulacean acid metabolism (CAM) plants inhabit areas with severe aridity and open their stomata at night to fix $\text{CO}_2$ in the form of malate, storing it until the following day.

1. a) If you compare a leaf section of a CAM succulent plant with that of a typical C3 mesophyte under the microscope, what specific histological adaptations would you expect to find regarding the proportion of water-storage parenchyma versus chlorenchyma, leaf thickness, and the organ's surface-to-volume ratio? Justify the adaptive value of each feature.
2. b) Justify at a cytological level why the parenchymal cells responsible for storing malate overnight require extraordinarily large vacuoles relative to the cytoplasmic volume.
3. c) Formulate a hypothesis explaining why succulent plants typically exhibit much slower growth and biomass accumulation rates than non-succulent plants, relating the histology of their mesophyll (resistance to gas diffusion) to their photosynthetic strategy.

In a dendrogeomorphological study on a mountain slope, cylindrical wood cores are extracted from several conifer specimens. The analysis reveals three histological anomalies:

1. From the year 2011 onwards, the rings of several trees on the middle section of the slope show marked asymmetry: they are much wider on the downslope-facing side than on the upslope-facing side.

2. In the 2005 ring, 90% of the specimens on the slope show an extraordinarily narrow ring throughout its entire perimeter, but completely concentric and symmetrical.

3. In the 1998 ring, a charred necrotic wedge appears, sealed by parenchymatous callus tissue and subsequent wood.

1. a) Deduce and interpret what geological, climatic, or ecological process occurred on the slope at each of the three indicated dates, justifying your conclusions.
2. b) Explain the biological mechanism by which the vascular cambium of a conifer produces asymmetrical "reaction wood" when the trunk loses its vertical orientation.
3. c) A construction company claims that the slope is completely stable and has been free of geological hazards for more than a century. Use the data from the analysis to technically refute this stance.

Mangroves inhabit the intertidal zone of tropical coastlines in hypoxic, highly saline muddy substrates.

1. a) In a field experiment, all the lenticels of the pneumatophores of a mangrove seedling are sealed with waterproof resin. After 48 hours, an almost total cessation of root absorption of potassium and nitrate ions is detected, despite the abundance of these nutrients in the mud. Justify this result physiologically, indicating which cellular respiration process and which transmembrane transport mechanism have been compromised by the lack of oxygen.
2. b) If you observe the root cortex of a mangrove under the microscope and compare it with that of a non-floodplain temperate forest tree, what specialized parenchymal tissue would you expect to see hypertrophied, and what are its biomechanical and respiratory functions?
3. c) Seawater has a markedly negative water potential due to dissolved salts. Reason what osmotic problem the cells of the endodermis and root parenchyma face in order to absorb water, and how they solve it at the vacuolar level to prevent plasmolysis.

A farmer conducts two propagation trials on his farm:

• Trial 1: He grafts scions of a select pear variety onto wild rootstocks. In one group of rootstocks, he carefully aligns the peripheral zones of the stem and the graft takes; in another group, the scions are inserted crosswise, leaving the cortex of the scion in direct contact with the central heartwood of the rootstock, and all of them dry out and die.

• Trial 2: He collects seeds from the pears obtained from the successful trees of Trial 1 and sows them, hoping to obtain saplings that produce the exact same fruit without needing to graft again.

1. a) Justify at a histological and cytological level why the union failed in the second group of Trial 1, and which meristem must match with millimetric precision for the continuity of the vascular tissues to be restored.
2. b) Reasonably refute the farmer's expectation in Trial 2, explaining the biological difference between vegetative reproduction (mitotic origin of the graft) and sexual reproduction (meiosis and recombination in seed formation).
3. c) If a boring insect bores into and destroys a complete ring of phloem around the base of the rootstock trunk (below the graft junction), predict what will happen in the short and medium term to the tree crown and to the roots.

In a wood technology laboratory, two identical cubic wood samples are studied (Sample A and Sample B):

• Sample A: Homoxylous woody system composed of 92% tracheids with bordered pits and very fine uniseriate parenchymal rays; density of 0.42 g/cm³.

• Sample B: Heteroxylous woody system with abundant large-lumen vessels, a high proportion of sclerenchyma fibers with densely lignified walls, and wide parenchymal rays; density of 0.88 g/cm³.

1. a) Deduce which major plant group (gymnosperms or dicotyledonous angiosperms) each sample belongs to, justifying your deduction based on the cell types observed in the secondary xylem.
2. b) Predict and justify which of the two woods will exhibit greater mechanical resistance to bending and impact, and which will have a higher risk of suffering embolism or cavitation in its conducting elements during a severe frost.
3. c) A cabinetmaker observes that a door built from solid wood of Sample B rubs against the frame during the humid winter months, but fits loosely in the dry summer. Explain the physicochemical cause of this phenomenon at the level of the plant cell wall and name which macromolecular component is primarily responsible for this hygroscopicity.

Topic Challenge. How does the vegetation in your environment withstand weather variations through its internal anatomy? As a consulting botanist, your mission is to link the tissue structure of accessible plants with the actual climatic conditions of your municipality.

1. Climatic data collection: Access the Met Éireann web portal and consult the average relative humidity and cumulative precipitation from your nearest weather station over the last $30$ days.

2. Home observation and measurement: Select two common plant samples from home (for example, a celery stalk and a bay leaf or succulent plant). Using your mobile phone camera and a millimeter ruler, estimate thicknesses (for example, leaf blades from $0.2$ mm to $0.5$ mm) and identify conducting tissues (xylem and phloem bundles), dermal tissues, and mechanical tissues (collenchyma or sclerenchyma) with the naked eye.

3. Reasoned technical report: Write a one-page report that includes a comparative table. Justify how the distribution and development of parenchyma, vascular tissues, and ground/support tissues allow each plant to respond to the aridity or humidity levels recorded by Met Éireann.

Evaluation criteria:

• Correct anatomical identification and functional justification of the different plant tissues observed.

• Rigorous integration of official meteorological data from Met Éireann into the argumentation.

• Technical quality, precision in estimates, and organizational clarity of the comparative report.