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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.

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.

The main differences concern cellular construction and the usual modes of obtaining food, movement and development; the main interdependence concerns transfers of matter and energy through ecosystems. The nineteenth-century animal–vegetable division is useful as a comparison of familiar forms, but it is not a complete modern classification of life.

A typical plant cell has a cell wall, a large central vacuole and, in photosynthetic tissue, chloroplasts. An animal cell lacks a cellulose wall and chloroplasts and has a different extracellular and tissue organization. Both possess a cell membrane, cytoplasm, a cell nucleus and, in their usual aerobic cells, mitochondria. Roots and other non-green parts of a plant need not contain functional chloroplasts; an erythrocyte's loss of its cell nucleus is likewise not a definition of all animal cells.^[ref]

Most green plants acquire carbon through photosynthesis, taking carbon dioxide from their surroundings and mineral nutrients and water chiefly through roots. Familiar animals obtain preformed organic material by feeding and digesting it or absorbing material derived from a host. Animals generally have conspicuous coordinated movement, and many have nervous systems and muscles; higher plants regulate growth and movement without such organs. These are useful generalizations with exceptions. Some plants depend on other organisms for carbon; some animals are permanently attached; and neither lack of locomotion nor a green appearance alone settles every microscopic classification. Fungi and bacteria should not be forced into one of these two kingdoms in a modern account.

One major interdependence is nutritional. Photosynthesis converts light input into chemical energy stored in organic material. Plant material feeds herbivores and, through them, carnivores; it also enters detrital food webs. An animal that eats an animal can therefore remain indirectly dependent on primary production by plants, algae or other photosynthetic organisms. Chemical elements pass between organisms, but energy is progressively dissipated as heat; it is not recycled in the same closed manner as an element.^[ref]

A second relation is gas exchange. In its simplest overall form, oxygenic photosynthesis may be represented byAerobic utilization of the resulting glucose has the complementary overall balanceThese balanced equations show complementary material transfers without making the underlying pathways literal reversals of one another. Plants also perform cellular respiration, by day as well as by night. They can therefore release carbon dioxide while consuming oxygen; a sufficiently illuminated leaf may simultaneously show net uptake of carbon dioxide because photosynthesis exceeds respiration. Stating that plants only purify air and animals only spoil it would erase this essential distinction between gross processes and net exchange.

Other relations close material cycles. Animals return organic matter through excretion and death; microbial decomposition and mineral transformations return nutrients in forms available for plant uptake. Most atmospheric nitrogen is not directly available to ordinary plants merely because animals have breathed or excreted near them: nitrogen fixation, ammonification and often nitrification involve particular microorganisms. Plants and animals also interact through pollination, seed dispersal, grazing, parasitism and shelter. Such relations may benefit both partners, one partner at the other's expense, or different partners under different conditions; interdependence is not universal mutual benevolence.

A closed illuminated aquarium containing plants and animals gives an instructive experiment, provided gas exchange with air, illumination, nutrient supply and organismal amounts are specified. Dissolved oxygen and carbon dioxide should be followed over light and dark periods. Survival alone is an ambiguous readout: microorganisms, stored food and air exchange may support it temporarily. A more controlled comparison includes illuminated and dark plant vessels and animal-only vessels. The expected alternation of net gas exchange makes the physiological distinction visible while exposing why no simple rule guarantees indefinite balance in any arbitrarily stocked closed jar.

In angiosperms, pollination transfers pollen from an anther to a stigma. Fertilization is a later fusion of gametes; seed development follows. Pollination may use animals, wind or water, so an insect visit is neither required for every flower nor sufficient evidence of fertilization.^[ref]