Mushrooms are not plants. They belong to the kingdom Fungi, a lineage that split from plants over a billion years ago and, according to molecular evidence, actually shares a more recent common ancestor with animals than with any plant. The confusion is understandable: mushrooms sprout from soil, stay rooted in place, and look more like a weird vegetable than like anything related to a beetle or a sea sponge. But once you look past the surface, fungi differ from plants in almost every fundamental way, from how their cells are built to how they eat, grow, store energy, and reproduce.
Why Fungi Were Ever Grouped with Plants
For centuries, Western naturalists divided living things into two big buckets: animals (things that move) and plants (things that don’t). Mushrooms don’t walk around, so they got filed with plants. Early taxonomists reinforced this by noting that fungi, like plants, have cell walls and produce spores. Linnaeus himself placed fungi in the plant kingdom in the 1700s, and that classification stuck in textbooks well into the twentieth century. It wasn’t until advances in molecular biology allowed researchers to compare protein and DNA sequences across organisms that the picture changed dramatically. By the early 1990s, analyses of multiple proteins showed that animals and fungi group together as a single evolutionary branch, excluding plants entirely.1PubMed. Animals and fungi are each other’s closest relatives: congruent evidence from multiple proteins This clade, now called Opisthokonta, means the mushroom on your pizza is a closer relative of yours than it is of the lettuce underneath it.2PubMed. Evolution: Divergent trajectories predate the origins of animals and fungi
That evolutionary kinship doesn’t mean fungi look or act like animals. Animals and fungi diverged into wildly different body plans and lifestyles. But the shared ancestry shows up in small biochemical details: both store energy as glycogen rather than starch, both lack the ability to photosynthesize, and both use similar signaling molecules in certain cellular pathways. Plants, by contrast, went their own way long before the animal-fungal ancestor even existed.
Cell Walls That Tell a Different Story
Both plants and fungi have rigid cell walls, which is one reason early scientists lumped them together. But the walls are made of completely different materials. Plant cell walls are built primarily from cellulose, a polysaccharide made of glucose chains. Fungal cell walls rely instead on chitin, the same tough polymer that makes up the exoskeletons of insects and crustaceans, along with glucans, mannans, and various glycoproteins.3PubMed. The structure and synthesis of the fungal cell wall Chitin is the second most abundant polysaccharide on Earth after cellulose, but plants don’t produce it.4PubMed Central. Chitin and chitin-related compounds in plant–fungal interactions
The differences go deeper than the wall. Inside the cell membrane, fungi use a sterol called ergosterol to maintain membrane fluidity, while animals use cholesterol and plants use phytosterols like sitosterol and stigmasterol. Ergosterol is so specific to fungi that many antifungal drugs work by targeting it, disrupting the fungal membrane without harming plant or animal cells. This biochemical quirk also matters to your immune system: the polysaccharides in fungal cell walls, including beta-glucans, chitin, and mannans, are recognized by specific receptors on human immune cells, triggering defensive responses like inflammation and pathogen clearance.5PubMed. Rethinking the immune recognition of cell walls in human fungal pathogens: Mechanisms, controversies and translational significance Your body evolved to identify fungal cell walls as foreign precisely because they’re built so differently from your own cells or from plant tissue.
How Fungi Eat
This is perhaps the most fundamental difference between fungi and plants. Plants make their own food through photosynthesis, capturing sunlight and converting carbon dioxide and water into sugars. Fungi cannot do this. They lack chlorophyll and chloroplasts entirely. Instead, fungi are heterotrophs: they get their carbon and energy by absorbing nutrients from their surroundings, feeding on other organisms or on dead organic matter.
But fungi don’t eat the way animals do, either. Animals typically ingest food first, then digest it internally. Fungi reverse the process. They secrete enzymes into their environment that break down complex organic molecules outside the fungal body, then absorb the resulting smaller molecules through their cell walls.6PubMed Central. Extracellular Enzyme Activities and Carbon/Nitrogen Utilization in Mycorrhizal Fungi Isolated From Epiphytic and Terrestrial Orchids Think of it as digesting your meal on the plate before bringing it to your mouth. This external digestion strategy is one reason fungi are so effective at decomposing tough materials like wood. Wood-decay fungi produce specialized enzymes that can break apart lignin, the rigid polymer that gives wood its strength, through oxidation reactions that few other organisms can manage.7PubMed Central. Lignocellulolytic Enzyme Production from Wood Rot Fungi Collected in Chiapas, Mexico, and Their Growth on Lignocellulosic Material
Energy Storage Sits Between Plants and Animals
How an organism stores energy for later use reveals a lot about its biology. Plants pack surplus sugars into starch, a polysaccharide with relatively sparse branching that’s well suited for long-term, slow-release energy storage. Animals store energy as glycogen, a densely branched molecule that can be broken down quickly when muscles need fuel. Fungi also store energy as glycogen, not starch, which is another molecular link to the animal kingdom. But fungal glycogen isn’t identical to the animal version. Structural comparisons show that fungal glycogen has branching characteristics intermediate between plant starch and animal glycogen, with a high proportion of short chains that allows moderately rapid energy release.8PubMed. Structural evolution and functional adaptation of energy-storage polysaccharides in animals, plants and fungi Fungi landed on their own solution: not quite as slow-burn as a potato’s starch granules, not quite as quick-access as your liver glycogen, but something in between.
Growing by Threads, Not by Leaves
When you see a mushroom, you’re looking at just the reproductive structure of the fungus, the equivalent of a flower on a plant. The actual body of most fungi is a sprawling network of microscopic filaments called hyphae, collectively known as the mycelium. This network can spread through soil, wood, or other substrates for meters in every direction, and it’s where the real work of feeding, growing, and exploring happens.
Hyphae grow at their tips in a way that has no parallel in plant biology. At the apex of each growing hypha sits a structure called the Spitzenkörper, a cluster of tiny vesicles that coordinates the delivery of wall-building materials to the tip, directing where and how the hypha extends.9PubMed. The Spitzenkörper: a choreographer of fungal growth and morphogenesis Computer simulations have confirmed that the Spitzenkörper acts as a kind of steering mechanism, controlling the shape and branching pattern of hyphae by regulating vesicle traffic.10PubMed. Analysis of the role of the Spitzenkörper in fungal morphogenesis by computer simulation of apical branching in Aspergillus niger Plants grow by cell division at their root tips and shoot tips too, but their growth structures and mechanisms are completely different. Fungal hyphae are tubular, often multinucleate (multiple nuclei sharing a single long cell), and can fuse with each other to create interconnected networks. Plant cells, by contrast, are discrete compartments with rigid walls separating each cell.
This networked growth pattern gives fungi extraordinary flexibility. A mycelial network can shuttle nutrients from one region to another, explore new territory in multiple directions at once, and rapidly colonize fresh food sources. It’s less like a tree growing upward and more like an expanding underground internet.
Reproduction and the Mushroom’s Real Job
Both plants and fungi use spores to reproduce, which is another superficial similarity that once encouraged the plant classification. But fungal reproduction works very differently. The two largest groups of fungi, the Ascomycota and the Basidiomycota, dominate the kingdom’s known species and have life cycles characterized by a haploid-dominant phase, meaning most of the organism’s life is spent with a single copy of its genetic material, punctuated by brief diploid stages during sexual reproduction.11PubMed Central. Diploid-dominant life cycles characterize the early evolution of Fungi Plants, in contrast, alternate between haploid and diploid generations in more complex ways, and the conspicuous parts of most plants (leaves, stems, flowers) are diploid.
The mushroom itself is essentially a spore-delivery device. When conditions are right, the underground mycelium channels resources into rapidly building a fruiting body. Research on mushroom development shows this involves an early phase of cell division and differentiation that lays out the body plan, followed by a phase of cell expansion, spore-producing meiosis, and sporulation.12Studies in Mycology. Lessons on fruiting body morphogenesis from genomes and transcriptomes of Agaricomycetes Detailed studies of specific mushroom species have found that the cap and stem actually divide labor: the stem serves as the main site for amino acid production and nitrogen metabolism, while the cap accumulates higher protein concentrations and coordinates carbohydrate metabolism, with materials being shuttled back and forth between the two.13PubMed Central. Comparative analysis of proteomes and transcriptomes revealed the molecular mechanism of development and nutrition of Pleurotus giganteus at different fruiting body development stages A mushroom is, in a sense, a tiny factory with specialized departments, assembled in days from an invisible underground network.
What Fungi Do for the Planet
Fungi occupy ecological roles that neither plants nor animals fill, and ecosystems would collapse without them. Their contributions fall into several broad categories.
As decomposers, fungi are the planet’s primary recyclers of dead plant material. Without wood-decay fungi breaking down fallen trees and leaf litter, carbon and nutrients would remain locked in dead biomass indefinitely. The enzymes fungi use for this work, particularly the ones that attack lignin, are among the few biological tools in nature capable of dismantling that polymer.
As mutualistic partners, many fungi form intimate relationships with living plants. Mycorrhizal fungi colonize plant roots and extend their hyphal networks far into the soil, vastly increasing the root system’s effective reach. The fungus delivers mineral nutrients like phosphorus to the plant, and in return receives sugars the plant produces through photosynthesis.14Molecular Plant. Nutrient Exchange and Regulation in Arbuscular Mycorrhizal Symbiosis The vast majority of land plants form some kind of mycorrhizal association. Without these fungal partners, most forests and grasslands would struggle to extract enough nutrients from poor soils.
As pathogens, fungi can also cause devastating diseases in plants, animals, and humans. Pathogenic fungi have evolved specialized infection structures called appressoria, which attach to a host’s surface and generate enough mechanical force to punch through cell walls and invade tissue.15PubMed Central. Appressoria-Small but Incredibly Powerful Structures in Plant-Pathogen Interactions These structures are responsible for major crop diseases like rice blast, rusts, and powdery mildews. Some fungal pathogens can form appressoria and penetrate host tissue even when their own cell division has been artificially blocked, indicating how robustly these infection mechanisms have been hardwired by evolution.16PubMed Central. Cell cycle and cell death are not necessary for appressorium formation and plant infection in the fungal plant pathogen Colletotrichum gloeosporioides
Lichens and the Blurred Boundaries
If fungi aren’t plants, what about lichens? Lichens are those crusty, leaf-like, or stringy growths on rocks, tree bark, and old fences. They look plant-like and even photosynthesize, but lichens are actually symbiotic partnerships between a fungus and one or more photosynthetic organisms, typically a green alga or a cyanobacterium.17PubMed. Evolutionary biology of lichen symbioses The fungal partner provides structure and protection, while the algal or cyanobacterial partner provides sugars through photosynthesis. Lichens aren’t plants, and they aren’t purely fungi either. They’re composite organisms that blur the lines between kingdoms. Their existence illustrates how creatively life can combine very different biological toolkits.
A Chemical Factory Like No Other
Fungi produce an extraordinary range of chemical compounds that plants and animals simply don’t make, or make only rarely. These secondary metabolites, molecules not strictly necessary for basic growth and survival, include some of the most important drugs in human medicine. Penicillin, the antibiotic that transformed twentieth-century medicine, comes from a mold. Cyclosporin, used to prevent organ transplant rejection, is a fungal product. Lovastatin, one of the first cholesterol-lowering statin drugs, was originally derived from a fungus.18PubMed Central. A Comprehensive Review of the Diversity of Fungal Secondary Metabolites and Their Emerging Applications in Healthcare and Environment
The diversity of fungal chemistry is staggering. Fungi produce polyketides, nonribosomal peptides, terpenes, and alkaloids through distinct biosynthetic pathways. Some of these compounds are beneficial to humans, and some are profoundly toxic. Ergot alkaloids, produced by fungi that infect grain, cause hallucinations and gangrene in people who eat contaminated bread. Aflatoxins, produced by certain molds that grow on stored crops, are potent carcinogens. Mycotoxins from various species contaminate food supplies and indoor environments worldwide.19PubMed. Biologically Active Secondary Metabolites from the Fungi This chemical versatility reflects the unique evolutionary pressures fungi have faced: competing with bacteria in soil, defending against being eaten, and breaking into or out of host organisms.
Do Fungi “Think”?
Fungi don’t have brains, neurons, or anything resembling a nervous system. But their mycelial networks show behaviors that, in any other context, might be called decision-making. When a fungal network colonizes a food source and is then given access to new resources in a different location, the network reorganizes itself in ways that aren’t random. Researchers studying mycelial networks found something striking: when a mycelium that had previously colonized a resource was allowed to regrow, it preferentially extended from the side that had originally been connected to the colonized resource. This memory-like behavior appears to help the organism rebuild damaged connections and grow toward areas where resources were previously abundant.20PubMed Central. Ecological memory and relocation decisions in fungal mycelial networks: responses to quantity and location of new resources
Nobody is claiming fungi are conscious. But these network-level behaviors, which include reallocating biomass from depleted regions to richer ones, choosing among multiple food sources, and maintaining efficient transport routes through the mycelium, suggest a form of biological computation that has no equivalent in the plant world. Plants respond to their environment too, bending toward light or growing roots toward water, but fungal networks process spatial information across large interconnected systems in ways that look more like problem-solving than simple tropism.
Fungi as Building Material
The unique properties of fungal mycelium have attracted growing interest from materials scientists and engineers. Mycelium naturally binds together loose substrates like sawdust, straw, or agricultural waste as it grows through them, forming a lightweight composite material that can be dried and shaped. Researchers have been testing dozens of fungal strains for their ability to produce these mycelium-bound biocomposites, evaluating properties like firmness, elasticity, and water resistance.21PubMed Central. Mycelium-Composite Materials-A Promising Alternative to Plastics? The appeal is obvious: the raw materials are cheap agricultural byproducts, the manufacturing process runs at room temperature using biological growth rather than industrial chemistry, and the finished product is fully biodegradable.
Companies are already selling mycelium-based packaging as a replacement for polystyrene foam, and experimental products include mycelium leather, insulation panels, and even structural building blocks. These materials align with circular economy principles because the fungus feeds on waste streams and the end product can be composted at the end of its life.22ACS Sustainable Chemistry & Engineering. Sustainable Mycelium-Bound Biocomposites: Design Strategies, Materials Properties, and Emerging Applications A plant can give you wood or cotton. An animal can give you leather or wool. Fungi are now offering a third option that doesn’t require harvesting a living organism, clear-cutting a forest, or drilling for oil. The kingdom that spent hundreds of millions of years quietly decomposing the world’s waste is now being recruited to replace the synthetic materials that the world doesn’t know how to decompose.