Exercises: Plant reproduction

Name the four whorls of a complete flower (calyx, corolla, androecium, and gynoecium) and state the parts and function of each one.

Differentiate between pollination and fertilisation, indicating what travels in each process, through where, and which floral structures are involved.

Which floral structure does the fruit develop from, and which does the seed develop from? Give an example of a fleshy fruit and a dry fruit, indicating which part we eat in each case.

Distinguish between a flower and an inflorescence, and give an example of an inflorescence (the capitulum of the sunflower, the spike of wheat, the umbel of the carrot), explaining what advantage grouping many small flowers can have.

Explain what hermaphrodite flowers and unisexual flowers are, and what monoecious and dioecious plants are, with an example of each case (maize; kiwi or holly).

What is nectar and which floral structures produce it (nectaries)? Explain the "contract" it establishes between the plant and the pollinator (food in exchange for pollen transport).

Name the parts of the pericarp of a fleshy fruit (exocarp, mesocarp and endocarp) using the cherry as an example, and explain the difference between a true fruit and a false one (a pseudofruit: the apple, the strawberry).

Explain what germination is and which three external conditions a seed needs (water, oxygen and a suitable temperature), discussing why seeds kept in a dry jar do not germinate.

Describe the reproductive cycle of an angiosperm from when a pollen grain reaches the stigma until the seed is formed: pollen germination, pollen tube, and double fertilisation (in a simplified way).

Compare the floral adaptations of anemophilous plants (wind-pollinated, such as grasses) and entomophilous plants (insect-pollinated): flowers, pollen, nectar, and efficiency of the process.

Look at the sectioned flower in the image. a) Indicate which numbers correspond to the androecium and gynoecium, and which parts form each one. b) Explain the function of structures 4 and 5 during reproduction. c) Describe what happens to structure 6 and the ovules it contains after fertilisation.

Many plants with hermaphrodite flowers cannot self-fertilise. Explain two mechanisms that prevent it (staggered maturation of stamens and stigma, genetic self-incompatibility) and discuss what evolutionary advantage preventing self-fertilisation has even though it risks ending up without seeds.

The orchid Ophrys mimics a female bee and attracts males, which pollinate while attempting to copulate. Explain what coevolution between flower and pollinator is, which traits of the flower (shape, colour, smell) take part in the deception, and why such extreme specialisation is at once an advantage and a risk.

Relate the structure of each diaspore to its disperser: the pappus of the dandelion and the wings of the maple fruit with the wind, and the coconut with water. Explain which traits (lightness, surface, buoyancy) each medium selects and why dispersing far from the mother reduces competition.

Dormancy synchronises germination with the favourable season. Explain three mechanisms that maintain it (a hard impermeable coat, chemical inhibitors, the need for a cold period — stratification) and which signal breaks each one, giving an example of a plant for each mechanism.

Some Mediterranean pines keep their cones closed until a fire passes (serotiny) and certain seeds only germinate with smoke. Explain what these adaptations to fire consist of and what advantage germinating just after a fire has (free soil, light, nutrient-rich ashes).

In an Iberian natural park, the diaspores of three unidentified plant species (A, B, and C) are analyzed:

• Diaspore A possesses a low-density, spongy pericarp with large air spaces and a waxy, hydrophobic outer cuticle.

• Diaspore B consists of a cypsela with a feathery pappus of high specific surface area relative to its mass (0.02 g).

• Diaspore C has a fleshy, sugary, bright reddish mesocarp, and a stony endocarp containing lignin that is resistant to acidic pH and proteases.

1. a) Deduce the dispersal syndrome (anemochory, endozoochory, or hydrochory) of each species, justifying what functional advantage each described anatomical adaptation provides.
2. b) Propose a representative species of the Iberian flora for each of the three described strategies.
3. c) If a dam is built that disrupts the seasonal flow and fragments the riparian forest, predict with reasoning which of the three populations will suffer greater genetic isolation and a shorter seedling recruitment distance from the parent plant.

To study the adaptation to aridity of an annual plant from southeastern Iberia, 400 freshly harvested mature seeds are divided into four equal batches subjected to different experimental conditions in Petri dishes at 22 °C:

• Batch 1 (Standard watering): Continuously moistened with 5 mL of distilled water. Result: 0% germination.

• Batch 2 (Scarification): Mechanical friction of the seed coat + 5 mL of distilled water. Result: 4% germination.

• Batch 3 (Simulated torrential rain): Continuous washing with running water for 48 h (equivalent to 75 mm of precipitation) followed by wet rest. Result: 88% germination.

• Batch 4 (Simulated light rain): Brief washing with 5 mL for 15 minutes (equivalent to 4 mm of precipitation). Result: 2% germination.

1. a) Interpret the results: what type of dormancy (physical due to seed coats or physiological due to soluble chemical inhibitors) does the species exhibit? Justify your answer by relating it to the hormonal balance between abscisic acid (ABA) and gibberellins.
2. b) From an evolutionary perspective, reason why a mutation that deactivated dormancy and allowed 100% of the seeds to germinate after a rain pulse of only 4 mm would be negatively selected (a lethal strategy) in this environment.
3. c) Explain the concept of a "persistent soil seed bank" and justify why fractional or staggered germination over several years constitutes a key adaptation to a highly unpredictable precipitation regime.

In a botany laboratory, three plant organisms (X, Y, and Z) are grown in isolated climate chambers where the relative air humidity is set at 45% and watering is supplied exclusively through subterranean capillary action, never wetting the surface of the plants:

• In species X, the dominant green individual is haploid. It develops mature male and female reproductive organs, but after several months, neither an embryo nor a diploid structure is observed.

• In species Y, the dominant photosynthetic plant is a diploid sporophyte producing meiospores. The spores germinate in the soil, forming an independent green laminar structure, but the formation of a new sporophyte never culminates under these growing conditions.

• In species Z, the dominant plant is a diploid tree with needle-like leaves and cones; it produces pollen grains that successfully germinate on the scales of female cones, fertilizing egg cells and generating viable seeds completely normally.

1. a) Deduce which plant group (bryophytes, pteridophytes, or gymnosperms) each organism belongs to and indicate which is the dominant generational phase (gametophyte or sporophyte) in each one.
2. b) Explain at the anatomical and physiological level what process fails in species X and Y under the test conditions, and what evolutionary innovation allows species Z to complete fertilization in the total absence of surface free water.
3. c) If a persistent environmental drought caused the disappearance of surface moist microhabitats, predict which specific generation (sporophyte or gametophyte) of species Y would act as the bottleneck that would cause its local extinction. Justify your deduction.

A rural municipality has two adjacent agricultural farms of 50 ha each: Farm A grows almond trees (Prunus dulcis) and Farm B grows barley (Hordeum vulgare). Over the last decade, the intensive use of broad-spectrum insecticides in the region has caused a 75% drop in wild pollinator populations (solitary bees, bumblebees, and syrphid flies).

1. a) Justifiably predict how the economic yield and production (in kg/ha) of each of the two farms will be affected, arguing on the basis of the floral morphology and the pollen transport mechanism of each crop.
2. b) The owner of Farm A considers purchasing and releasing hives of the domestic honeybee (Apis mellifera) as the sole solution. Critically evaluate this proposal: reason two ecological or biological limitations of relying solely on a single domestic pollinator species compared to the wild community.
3. c) Design three specific agroecological measures to be implemented in the margins of Farm A aimed at recovering and sustaining wild pollinators over the long term, assessing the cost or spatial commitment that each would require of the farmer.

In a Neolithic archaeological site, wheat samples dated to two historical moments are analyzed:

• Sample 1 (10 500 years ago): 82% of the ear remains correspond to rachis nodes with smooth spontaneous disarticulation scars (brittle rachis) and caryopses with tightly adhering glumes.

• Sample 2 (7 000 years ago): 96% of the ears show rachises with irregular fractures caused by mechanical threshing and naked grains that easily separate from the chaff.

1. a) Interpret the change observed between both samples in terms of artificial selection: why did early farmers unconsciously select mutants with a non-brittle rachis and free grain?
2. b) Justify why the phenotype of Sample 2 would represent a clearly disadvantageous (suicidal) mutation for the plant in a natural ecosystem, arguing the absolute dependence that modern wheat has on human sowing to prevent intraspecific competition and ensure its dispersal.
3. c) Unlike annual cereals domesticated by seed, many fruit trees domesticated in Antiquity (such as the fig, olive, or grapevine) were propagated using cuttings and grafting. Justify the genetic advantage that asexual propagation offered for preserving desired traits compared to seed reproduction in allogamous and heterozygous species.

A coastal conservation team attempts to eradicate two invasive species in a dune system:

• Case 1: The sour fig (Carpobrotus edulis), a creeping succulent plant, is mechanically crushed and shredded using heavy machinery 'in situ'. Four months later, the plant's cover density has tripled throughout the treated area.

• Case 2: Pampas grass (Cortaderia selloana), a giant dioecious tussock grass, is cut down to ground level just at the end of summer, removing the adult specimens. The following spring, tens of thousands of seedlings sprout within a radius of more than 2 km downwind.

1. a) Justify the failure of both interventions, explaining which reproductive strategy (sexual or asexual) and which type of propagule or diaspore each species exploited to react with such vigor.
2. b) Design a methodological modification for the eradication of sour fig that prevents the dispersal of vegetative propagules and ensures the definitive elimination of the biomass bank without eroding the dune.
3. c) Propose three strictly reproductive biological criteria (relating to pollination type, fecundity rate, dispersal, or seed persistence) that justify legally prohibiting the sale of an ornamental plant in gardening before it demonstrates its invasive potential.

A farmer grows two commercial varieties purchased from a nursery in neighboring vegetable plots: F1 hybrid zucchini (obtained from crossing two homozygous pure lines to achieve straight and very vigorous fruits) and traditional tomatoes of an old local variety. At the end of the season, the farmer collects seeds from the best fruits of both crops and sows them the following season:

1. a) When harvesting the second-generation (F2) zucchini crop, the farmer discovers with surprise enormous heterogeneity: bitter and wrinkled zucchini, others deformed, and plants with yields far lower than the previous year. Justify this phenomenon genetically by applying Mendel's laws and the concepts of heterosis (hybrid vigor) and allelic segregation in the F2.
2. b) In contrast, the tomato plants obtained from their own seeds faithfully retain the organoleptic quality and uniformity of the previous harvest. Justify this difference based on floral morphology (cleistogamy or inserted stigma) and the predominant pollination strategy (autogamy versus allogamy) of each species.
3. c) Evaluate the socioeconomic and ecological impact of commercial agriculture relying almost exclusively on F1 hybrid seeds: contrast the farmer's annual dependence on seed companies with the loss of agrobiodiversity when facing unforeseen environmental stresses.

An international plant genetic resources center receives two plant germplasm collections: Batch 1 contains seeds of wild lentil (Lens culinaris) and Batch 2 contains seeds of avocado (Persea americana). The standard seed-bank protocol, such as that of Svalbard, is applied to both batches: forced dehydration down to 5% water content and subsequent freezing at -18 °C.

1. a) After one year of storage, germination is tested: Batch 1 shows 95% viability, while in Batch 2 viability is 0% and the embryos show widespread cell lysis. Justify this result physiologically by distinguishing between orthodox and recalcitrant seeds, and explaining what cellular damage sub-zero cold causes in plant tissues that are not desiccation-tolerant.
2. b) Design two alternative 'ex situ' conservation strategies to maintain the genetic diversity of Batch 2 over the long term, taking into account that its seeds do not tolerate conventional storage.
3. c) In the context of the current climate emergency, argue why conserving wild and traditional varieties with lower immediate yields in germplasm banks is biologically more valuable for the future of humanity than exclusively preserving the few hyper-productive commercial varieties that dominate today's market.

Unit challenge. Have you ever thought about how plants coordinate fruit ripening to ensure dispersal? Turn your kitchen into a botanical laboratory to analyse it.

1. Reproductive inventory: Select two plant specimens from your pantry: one resulting from sexual reproduction (with seeds, such as a tomato or pepper) and another from asexual vegetative propagation (a tuber or bulb). Describe their reproductive structures and how they give rise to a new individual on an index card.

2. Phytohormone bioassay: Get two identical immature samples (green bananas or tomatoes). Place one in a sealed bag together with a ripe fruit (a large ethylene emitter) and leave the control in the open air. Record changes in texture and colour every $12.0$ hours for $48$ hours, rating them on a ripening scale from $1.0$ to $5.0$.

3. Reasoned report: Design a short report (comparative table and conclusions) explaining the physiological effect of ethylene compared to auxins or gibberellins, and argue the adaptive advantage of alternating sexual and asexual reproduction in angiosperms.

Assessment criteria:

• Accurate anatomical identification of the reproductive structures analysed.

• Methodological rigour in the systematic recording of bioassay data.

• Biological justification of the role of phytohormones and the adaptive success of plant reproduction.