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A plant that produces flowers and encloses its ovules within an ovary.

Asgard archaea are an archaeal group important in studies of eukaryotic origins. Molecular comparisons connect their genes to the nuclear lineage of eukaryotes, rather than making modern Asgard archaea the direct ancestors of modern animals or plants. See the original analysis cited under two-domain system.

The cell nucleus is a membrane-bounded compartment characteristic of eukaryotic biological cells, containing most of the cellular DNA in a typical nucleated biological cell. Its double nuclear envelope separates this DNA from the surrounding cytoplasm. Most does not mean all: mitochondria and chloroplasts retain their own DNA. Nor does every specialized eukaryotic biological cell retain a cell nucleus: mature mammalian red blood cells have lost theirs. Prokaryotes organize their DNA in a nucleoid without a nuclear envelope, and still have ribosomes. See NHGRI: nucleus, ^[ref] and Clark, Douglas and Choi, Biology 2e, 40.2: mature mammalian red blood cells.

Follow one nuclear protein-coding gene: its DNA is used in transcription to make RNA; after the appropriate processing, messenger RNA is exported through a nuclear pore complex; a ribosome in the cytoplasm then uses it in biological translation to build an amino acid chain. The DNA remains in the cell nucleus. The RNA is a template, not the material converted into a protein. The ribosome diagram below follows this route; gene expression gives a complementary nucleotide sequence example. See Alberts et al.: selective RNA export and protein import.

This architecture makes gene expression interesting in a spatial way: nuclear transcription and cytoplasmic biological translation are separated by a selective boundary. A messenger RNA can be processed before export and use, while proteins needed for nuclear functions travel inward. Inside the cell nucleus, the cell nucleolus helps assemble ribosome subunits for export. Thus the compartment housing most DNA also helps build the machinery that interprets its messages elsewhere.

Robert Brown described and named the cell nucleus in his account of plant biological cells, read to the Linnean Society in 1831 and published in its Transactions in 1833. This historical observation identified a recurring structure, without establishing its later-understood role in DNA storage or gene expression. See Brown, On the Organs and Mode of Fecundation in Orchideæ and Asclepiadeæ, Transactions 16, 685–738 (1833). This is a documented contribution rather than a claim that no one had observed the structure earlier.

Cellular respiration transfers energy from chemical fuels into usable forms including ATP. Oxidation of the fuel is coupled to reduction of an electron acceptor; the energy transfer can support ATP synthesis. Plants carry out cellular respiration as well as photosynthesis. Their biological cells need usable energy in darkness too, and a seed can draw on stored fuel during germination.^[ref][ref]

Follow three different accounts: carbon moving from fuel to products, Electrons moving to an electron acceptor, and energy becoming available for cellular work. They are connected, but an oxygen uptake measurement alone does not identify the fuel or count the ATP made. The pea seed respirometry experiment makes this distinction visible with simple apparatus.

A chloroplast is a compartment in photosynthetic plant and algae biological cells. Its internal thylakoids carry out light-dependent reactions; its surrounding chloroplast stroma contains the enzymes of the Calvin cycle. Cyanobacteria perform oxygenic photosynthesis without chloroplasts. See Alberts et al., Molecular Biology of the Cell: chloroplasts and photosynthesis.

Transfer of pollen between different individuals of the same species; compare self-pollination.^[ref]

A flower can contain both reproductive groups and still receive pollen from another individual. Conversely, transfer between two flowers on one plant does not by itself imply cross-pollination. Following the donor and recipient individuals makes pollination useful for studying genetics: organ position alone cannot identify whose alleles meet at fertilization.

Cyanobacteria are bacteria that perform oxygenic photosynthesis: light-dependent reactions release oxygen from water. Their photosynthetic machinery operates on thylakoid membranes without a chloroplast or a cell nucleus. This is a particularly useful counterexample to equating photosynthesis with being a plant. Chloroplasts descend from an ancestral cyanobacterial partner through endosymbiosis; present-day cyanobacteria are independent descendants, rather than chloroplasts waiting to become plants. See Alberts et al., Molecular Biology of the Cell: chloroplasts and cyanobacteria.

In the common angiosperm arrangement, one sperm fuses with the ovum to form a diploid zygote; another fuses with the central biological cell's two polar cell nuclei, initiating endosperm with three chromosome sets. The zygote develops into a plant embryo, and the ovule develops into a seed.^[ref]

For an original counting example, stipulate a plant with $2n=14$ chromosomes and the common two-polar-nucleus arrangement. Each haploid sperm and ovum contributes $n=7$. The zygote therefore has $7+7=14$ chromosomes, while the initiating endosperm cell nucleus has $7+7+7=21$. The maternal seed coat remains derived from $2n=14$ plant tissue; the whole seed has no single uniform chromosome count. This is bookkeeping for a stipulated example, not identification of a particular species. It follows by counting the chromosome sets entering each fusion, rather than assigning one count to everything enclosed by a seed coat.

Endosymbiosis is an ecological association in which one organism, the endosymbiont, lives inside another. The definition specifies location, not that both partners benefit, nor that the internal partner has become an organelle. An intracellular association can remain an association between distinct organisms. Endosymbiotic theory addresses a different question: how particular ancestral bacteria became inherited, functionally integrated organelles during evolution.

The striking consequence is that a eukaryotic biological cell can combine histories: its nuclear lineage and its mitochondria need not have the same ancestry. Mitochondria have bacterial ancestry related to Alphaproteobacteria; the plastids of plants and many algae, including chloroplasts, have ancestry within cyanobacteria. These are ancestral lineages, not a claim that a named living bacterial species is the literal ancestor. Present-day bacteria and organelles have both evolved since their shared ancestors. ^[ref]

A eukaryote is an organism belonging to the lineage characterized by a membrane-bounded cell nucleus and extensive cellular compartmentalization. A nuclear envelope surrounds the nuclear genome; nuclear chromosomes are typically linear. Ribosomes perform biological translation outside the cell nucleus. See OpenStax Microbiology, 3.4: nuclear organization and protein synthesis. Specialized biological cells can lose their cell nucleus, as mature mammalian red blood cells do, while remaining parts of eukaryotic organisms; see OpenStax Biology 2e, 40.2: mature red blood cells.

Yeasts, plants and animals are all eukaryotes. Typical cells in these groups have mitochondria, but chloroplasts belong to photosynthetic plant and algae lineages, not to every eukaryote. A cell wall occurs in typical plant and fungus cells but is absent from animal cells. These are comparisons of representative cells, not a checklist every specialized cell must satisfy. See OpenStax Biology 2e, 4.3: nucleus, organelles and representative cells.

The interesting feature is controlled separation: cell membranes and organelle membranes let different cellular processes occupy distinct compartments while membrane transport connects them. The evolution of eukaryotes also involved partnership: mitochondria descend from a bacterial endosymbiosis. Modern phylogenetics supports an archaeal ancestry for the nuclear lineage, alongside this bacterial contribution; see two-domain system. Thus the history of a whole eukaryote cannot be read as the history of just one of its genes.

A fungal cell wall contains structural polysaccharides including chitin and glucans. Its composition and organization vary among fungi; the diagram illustrates a composite rather than a pure chitin shell. A fungus sharing a cell wall with a plant does not imply shared wall chemistry.^[ref]

A fungus is a eukaryote that obtains nutrients by absorption rather than photosynthesis. Fungi include unicellular yeasts and multicellular forms. Their cell walls differ from bacterial peptidoglycan walls; having a cell wall does not make a fungus a plant. See OpenStax Biology 2e, 24.1: fungal nutrition and organization.

The plant life-cycle generation that produces gametes. In an angiosperm, male gametophytes are pollen grains and the female gametophyte is the embryo sac.

A hypertonic solution favors initial net water loss under the reference conditions above. In a walled plant cell, enough loss can produce plasmolysis.

Independent assortment means that transmission at different genetic loci has conditional independence given the parental genotype. With fair Mendelian segregation, a parent heterozygous at two genetic loci, $AaBb$, produces $AB,Ab,aB,ab$ gametes with probability $1/4$ each. For example,Here $AB$ labels an allele combination; it does not assert that the two genetic loci lie on the same chromosome.

The physical model is stochastic orientation of different homologous chromosome pairs toward the two poles of the spindle apparatus in meiosis I. One pair's orientation does not determine another's. For two pairs, one division can send $A,B$ together and $a,b$ together; another can send $A,b$ together and $a,B$ together. See The Process of Meiosis (§11.1, Metaphase I and Figure 11.5: random orientation of chromosome pairs).

With $k$ distinguishable homologous chromosome pairs, there are $2^k$ choices of parental homologous chromosome origin for a product at a specified pole, each with probability $2^{-k}$ under this model. For $k=3$, enumerate $MMM,MMP,MPM,MPP,PMM,PMP,PPM,PPP$, where M and P mark parental origins. Whole-pole reversal pairs complementary choices, leaving $2^{k-1}$ unordered partitions for $k\ge 1$. For $k=2$ there are four choices for a specified pole but only the two relative orientations drawn. This counts possible products across meioses, not eight different products of one division when $k=3$. Identical alleles at scored genetic loci can also make different parental-origin choices observationally indistinguishable.

Independent assortment of different chromosome pairs does not imply independence of every pair of genes. Nearby genetic loci on the same chromosome can show genetic linkage. Chromosomal crossover can create new haplotypes, but does not automatically erase genetic linkage. See Laws of Inheritance (§12.3, Law of Independent Assortment and Linked Genes Violate the Law of Independent Assortment). In the ordinary fair two-locus model with parental haplotypes $AB/ab$ and recombination fraction $r$,Thus $r=0$ gives only the two parental haplotypes, while $r=1/2$ gives four equal probabilities. A recombination fraction of $1/2$ alone does not establish that the genetic loci lie on different chromosomes.

A motivating experiment appears in Experiments in Plant Hybridization by Gregor Mendel (§9, “The Reproductive Cells of the Hybrids”). In peas, hybrids for round versus wrinkled seeds and yellow versus green seeds were crossed reciprocally with constant wrinkled-green plants. The readout was visible seed shape and color: hybrid seed parents gave 31 round-yellow, 26 round-green, 27 wrinkled-yellow and 26 wrinkled-green; the reciprocal cross gave 24, 25, 22 and 27.

The apparatus was controlled pollination: opening immature flowers, removing stamens with forceps, and applying selected pollen to the stigma. Two-year trials established constant parental characters; greenhouse pots checked interference from insects in outdoor beds. Reciprocal crosses tested both transmission directions. These procedures are described in Experiments in Plant Hybridization by Gregor Mendel (§§2–3).

With complete dominance at both genetic loci, this test cross makes each phenotype report one hybrid gamete combination. The ideal expected values are $110/4=27.5$ and $98/4=24.5$ per class. The observations support four transmitted combinations, approximately equally frequent. They do not directly image chromosome orientation or prove independence for all genes; failed development and seed germination also limit the inference, as §9 notes.

A plant organ; a cotyledon is an embryonic leaf.

A plant used in the lettuce red and far-red germination experiment.