Difficulty: Introductory | Prerequisites: Basic cell biology (mitosis, meiosis, ploidy levels)
This lab sits at the start of the plant biology sequence in B130. It introduces how plants are classified, how they differ from animals, and why the move from water to land was such a big deal evolutionarily. You need a working understanding of mitosis, meiosis, and the difference between haploid and diploid before this makes sense. If those are shaky, revisit them first.
Plants are far more diverse than trees and flowers. The group spans green and red algae, liverworts, mosses, ferns, lycophytes, gymnosperms, and angiosperms. All plants alternate between a haploid gametophyte stage (which makes gametes) and a diploid sporophyte stage (which makes spores). The evolutionary transition from water to land drove the development of key adaptations: cuticles to prevent water loss, stomata for gas exchange, vascular tissue for internal transport, and seeds and pollen for reproduction without standing water.
Alternation of generations
A life cycle in which the organism passes through two distinct multicellular stages, one haploid and one diploid, to reproduce sexually. Think of it as the plant toggling between two separate "adult" forms, each with a different job.
Gametophyte
The haploid (n) life stage of a plant. It produces gametes (sperm and eggs) by mitosis. In simple terms, this is the stage that makes sex cells.
Sporophyte
The diploid (2n) life stage of a plant. It produces single-celled spores by meiosis. In simple terms, this is the stage that results from fertilisation and makes spores.
Gametes
Haploid sex cells (sperm and eggs) produced by the gametophyte via mitosis. When two gametes fuse, they form a zygote.
Spores
Haploid single cells produced by the sporophyte via meiosis. Each spore can grow into a new gametophyte. Not the same as seeds: spores are single cells with no nutrient store or protective coat.
Zygote
A fertilised egg (diploid). The zygote develops into the sporophyte. Think of it as the starting point of the diploid generation.
Haploid (n)
Having one set of chromosomes. The gametophyte stage and all gametes are haploid.
Diploid (2n)
Having two sets of chromosomes. The sporophyte stage and the zygote are diploid.
Cuticle
A waxy, waterproof layer secreted by the exposed cells of a plant. It prevents water loss through evaporation. If you drop water on a surface with a cuticle, the water beads up rather than being absorbed.
Stomata (singular: stoma)
Pores on the plant surface that allow gas exchange. Each stoma is surrounded by a pair of guard cells that can open or close the pore. In simple terms, stomata are adjustable breathing holes.
Guard cells
Specialised cells flanking each stoma. They swell or shrink to open or close the pore, regulating water loss and CO2 intake.
Xylem
Vascular tissue that carries water and minerals upward through the plant. Xylem cell walls are reinforced with lignin, making them strong and rigid. Under a microscope, xylem cells appear large, empty, and often stained red.
Phloem
Vascular tissue that transports sugars and nutrients throughout the plant. Phloem cells are smaller than xylem cells and have green or blue-stained walls (no lignin). Xylem and phloem are always found adjacent to each other.
Lignin
A rigid polymer that strengthens xylem cell walls. Wood is made of layers of lignified xylem cells. Think of lignin as the structural scaffolding that lets vascular plants grow tall.
Pollen
A drastically reduced male gametophyte, just two cells. Small enough to be carried by wind or animals, pollen delivers sperm to the egg via a pollen tube, removing the need for standing water.
Seed
A structure containing a sporophyte embryo, a store of nutritive tissue, and a tough protective coat. Seeds let the embryo survive outside the parent plant, travel to new habitats, and germinate under less-than-ideal conditions.
Heterospory
The production of two different sizes of spore: microspores (which grow into male gametophytes) and megaspores (which grow into female gametophytes). Homosporous plants produce spores of only one size.
Microphyll
A small, simple leaf with a single vein. Found in lycophytes.
Macrophyll
A larger, more complex leaf with multiple veins. Found in ferns, gymnosperms, and angiosperms.
Plants and animals share the same basic survival needs, but they meet them differently in three key ways.
Energy and nutrients: Animals get both from a single source (food). Plants get energy from sunlight via photosynthesis and absorb nutrients separately from their surroundings (soil, water).
Mobility: Most animals move freely. Plants are effectively sessile, which means every structural and reproductive adaptation must work without locomotion.
Reproduction and ploidy: In animals, only the gametes (eggs and sperm) are haploid; the adult body is diploid. In plants, both a haploid multicellular stage (gametophyte) and a diploid multicellular stage (sporophyte) exist as mature organisms.
All plants cycle between two multicellular life stages:
The gametophyte (haploid, n) produces gametes (sperm and eggs) by mitosis.
Sperm and egg fuse at fertilisation to form a zygote (diploid, 2n).
The zygote develops into the sporophyte (diploid, 2n).
The sporophyte produces spores by meiosis.
Each spore germinates into a new gametophyte, and the cycle repeats.
There is no single "adult" stage in the animal sense. Both the gametophyte and the sporophyte can be considered mature forms. Which stage dominates varies by plant group: in mosses and liverworts, the gametophyte is the larger, longer-lived stage; in ferns, gymnosperms, and angiosperms, the sporophyte dominates.
Land offers two main advantages over an aquatic environment:
More light: Even very clear water filters out a substantial amount of sunlight. On land, photosynthetic surfaces receive more energy.
Better gas access: CO2 and O2 are present in higher concentrations in air and diffuse more readily through air than through water. Plants need CO2 for photosynthesis and O2 for cellular respiration.
The transition from water to land introduced several problems that drove the evolution of new adaptations:
Water loss: Without surrounding water, exposed plant tissues lose moisture rapidly through evaporation. The cuticle evolved to address this.
Gas exchange: A waterproof cuticle blocks gas diffusion too. Pores and, later, stomata with guard cells evolved to let CO2 in and O2 out without excessive water loss.
Internal transport: In water, every cell is bathed in nutrients. On land, only root cells contact the soil. Vascular tissue (xylem and phloem) evolved to move water, minerals, and sugars throughout larger plant bodies.
Structural support: Water provides buoyancy. On land, plants need rigid internal support to stand upright. Lignin in xylem cell walls provides this.
Reproduction without standing water: Ancestral plant sperm had to swim through water to reach eggs. Pollen and pollen tubes evolved in seed plants to deliver sperm without free water.
Embryo protection: In water, zygotes develop in the surrounding environment. On land, developing embryos are vulnerable to desiccation. Many land plant groups retain and nourish the sporophyte embryo inside the parent gametophyte or sporophyte.
Students often think spores and seeds are the same thing. They are not. A spore is a single haploid cell with no protective coat or nutrient store. A seed contains an embryo, nutritive tissue, and a tough outer layer.
Students often assume the "plant" they see is always the sporophyte. In mosses and liverworts, the dominant visible structure is the gametophyte. The sporophyte is a small structure attached to or growing from the gametophyte.
Students sometimes confuse meiosis and mitosis in the plant life cycle. Meiosis produces spores (in the sporophyte). Mitosis produces gametes (in the gametophyte). This is the reverse of what happens in animals, where meiosis produces gametes directly.
Students often think all plants have vascular tissue. Mosses, liverworts, and algae lack vascular tissue entirely. They absorb water and nutrients across their surfaces.
True or false: In plants, meiosis produces gametes. False. Meiosis produces spores (in the sporophyte). Gametes are produced by mitosis in the gametophyte.
Fill in the blank: The waxy waterproof layer on a plant surface is called the ___. Cuticle.
True or false: Mosses have vascular tissue. False. Mosses are non-vascular.
Fill in the blank: The two types of vascular tissue are ___ (carries water and minerals) and ___ (carries sugars). Xylem and phloem.
True or false: Both the gametophyte and sporophyte can be considered mature adult stages. True.
Q: Describe the alternation of generations life cycle. What is the ploidy of each stage, and what does each stage produce?
A: The gametophyte is haploid (n) and produces gametes (sperm and eggs) by mitosis. Gametes fuse at fertilisation to form a diploid (2n) zygote, which develops into the sporophyte. The sporophyte produces haploid spores by meiosis. Each spore germinates into a new gametophyte.
Q: Name two advantages that land environments offer plants over aquatic environments.
A: Greater availability of light for photosynthesis (water filters out light), and higher concentrations of CO2 and O2 that diffuse more readily through air than water.
Q: What problem does a cuticle solve, and what new problem does it create?
A: The cuticle prevents water loss by evaporation. However, it also blocks gas exchange (CO2 in, O2 out), which is why pores and stomata evolved.
Q: Explain why vascular tissue allows plants to grow larger.
A: Vascular tissue (xylem and phloem) transports water, minerals, and nutrients quickly over long distances. Without it, every cell must be close enough to the surface to absorb what it needs directly, which limits body size.
Q: Compare free-swimming sperm and pollen as reproductive strategies. What advantage does pollen confer?
A: Free-swimming sperm require standing water to reach the egg. Pollen is a reduced male gametophyte carried by wind or animals, delivering sperm via a pollen tube. This removes the dependence on free water for fertilisation, a major advantage in dry terrestrial environments.
⚠️ The alternation of generations is the single most important concept in this lab. Expect exam questions that ask you to trace ploidy through the cycle and identify which process (mitosis or meiosis) occurs at each transition.
⚠️ You will almost certainly be asked to connect specific adaptations (cuticle, stomata, vascular tissue, seeds, pollen) to the selective pressures of terrestrial life. Be able to name the problem each one solves.
⚠️ Know which plant groups are vascular and which are not. This is a fundamental division that affects body size, leaf type, and reproductive strategy.
This material connects directly to the Plant Reproduction lab, which covers the life cycles of mosses, ferns, gymnosperms, and angiosperms in greater detail. The character traits studied here (cuticle, stomata, vascular tissue, pollen, seeds) reappear as the basis for phylogenetic analysis and evolutionary tree construction. Understanding alternation of generations is also essential for genetics topics later in the course, since the ploidy shifts mirror meiosis and fertilisation concepts from earlier chapters.
Plant diversity, B130, BIOL 130, University of Michigan, alternation of generations, gametophyte, sporophyte, haploid, diploid, gametes, spores, zygote, cuticle, stomata, guard cells, stoma, xylem, phloem, lignin, vascular tissue, non-vascular plants, pollen, pollen tube, seed, heterospory, homospory, microspore, megaspore, microphyll, macrophyll, bryophytes, mosses, liverworts, ferns, lycophytes, gymnosperms, angiosperms, green algae, land plants, terrestrial adaptations, plant evolution, phylogenetic tree, character matrix, parsimony