A potato is a stem. Specifically, it’s a stem tuber, the swollen tip of an underground stem called a stolon, which the plant uses to store food.1Carnegie Museum of Natural History. Potatoes, Sweet Potatoes, and Yams: What’s the Difference? Calling it a root vegetable is a kitchen habit, not a botanical one, and the label lumps together foods that come from quite different plant parts.2Food Unpacked. Root vegetables and tubers Its look-alike, the sweet potato, really is a root, so two foods with nearly the same name sit on opposite sides of the line.
Where a Potato Actually Grows on the Plant
Potatoes form on stolons, stems that grow sideways through the soil instead of upward.3PubMed. The role of the potato (Solanum tuberosum) CCD8 gene in stolon and tuber development Stolons sprout from buds at the nodes near the base of the main stem, just as branches do above ground.4PubMed. The role of the potato (Solanum tuberosum) CCD8 gene in stolon and tuber development The tip of a stolon swells into a potato, so a potato is really a thickened stretch of underground stem.5eXtension. Tubers and Root Vegetables…What is the difference? (RE: Sweetpotatoes) Botanists call it a stem tuber: a stem that has become a food store.6PotatoPro. Sweet Potato vs Potato: Are They Really the Same Crop?
Dig up a plant and you can see this. The potatoes hang off pale, string-like stolons, not off the fine hairy roots. Pull the whole plant and you’ll usually find several potatoes attached at the ends of these stems, each at a different size.
That size difference comes from how the potato forms. Early in the season the stolons run outward just below the surface, each one a thin, pale stem with tiny scale leaves along it. When the plant sends its swelling signal, the tip of a stolon stops lengthening and starts to thicken. Cells there divide and fill with starch, and over several weeks the tip balloons into a potato. Each stolon tip makes one potato, which is why a single plant ends up with a cluster.
The plant also has true roots, and they look different. They’re many thin, widely branching roots that grow from the stems.7Taylor & Francis Online. Root development in potato and carrot crops – influences of soil compaction The swelling happens only on the stolons, never on these roots. Botanists classify plant parts by their structure, not by where they grow.
The Clues on Your Kitchen Potato That Prove It Is a Stem
The eyes on a potato are buds sitting at nodes, the same spots where leaves and branches grow on any stem. True root tubers have no nodes, no internodes and no tiny scale leaves.8Wikipedia. Tuber A root tuber can only sprout from crown tissue at the end that was attached to the old plant.9Wikipedia. Tuber So a potato sprouting from eyes all over its surface is behaving like a stem.
You can check this on a sprouting potato in your pantry. Look at how the eyes are spaced: they follow a spiral around the surface, like leaves spiraling around a stem, not a random scatter. Under or beside each eye there’s often a faint, eyebrow-like ridge, the scar of a tiny scale leaf, the same kind found on stolons. The end that was attached to the stolon, the stem end, has a small scar or nub and few eyes. The opposite end usually has the most eyes crowded together, because that was the growing tip. A carrot or sweet potato has no such spiral of buds, and a sweet potato sprouts mostly from one end, so seeing the pattern is a quick sign you’re holding a stem.
The buds are also what make a potato so good at starting over. Each eye can hold more than one bud, so if the main bud is damaged, a side bud in the same eye can start growing a few weeks later.10PubMed. Mint essential oil can induce or inhibit potato sprouting by differential alteration of apical meristem These sprouts let a stored potato quickly grow into a new plant at the start of the next season.11The potato: botany, production and uses. Tuber development.
That’s why gardeners plant pieces of potato rather than seeds. A piece can grow into a full plant as long as it carries at least one eye, since the bud inside the eye produces the new shoot. The flesh around it feeds the young sprout until it has leaves of its own.
Green skin is another stem-like trait. A potato left in light turns green because it makes chlorophyll, as stems and leaves do. The greening comes with a rise in solanine and related bitter compounds that can upset the stomach.12PubMed Central. Light Regulation of Chlorophyll and Glycoalkaloid Biosynthesis During Tuber Greening of Potato S. tuberosum Store potatoes in the dark and cut away green patches before cooking.
Why Cooks Call It a Root Vegetable
In everyday cooking, “root vegetables” covers potatoes, carrots, turnips and other underground foods.13Food Unpacked. Root vegetables and tubers Botanically, carrots and turnips are enlarged taproots, while potatoes are swollen stems.14Food Unpacked. Root vegetables and tubers
The word tuber is used loosely in everyday talk, sometimes applied to any starchy underground food, including sweet potatoes. Shop shelves and recipes group foods by how they’re cooked and eaten, not by which plant organ they are.
Sweet Potatoes Really Are Roots
The part of a sweet potato we eat is a root, made entirely of root tissue.15eXtension. Tubers and Root Vegetables…What is the difference? (RE: Sweetpotatoes) It forms when roots growing from the base of the plant’s stem swell with stored food.16PotatoPro. Sweet Potato vs Potato: Are They Really the Same Crop? Botanists call it a root tuber, while a regular potato is a stem tuber.17eXtension. Tubers and Root Vegetables…What is the difference? (RE: Sweetpotatoes)
Gardeners use this to grow slips. Set a sweet potato in water or damp soil in a warm spot, and after a few weeks leafy shoots appear, mostly from the end that was attached to the plant. Once the slips are long enough, they’re twisted off and planted, and roots from the base of each slip swell into new sweet potatoes. A potato is different: the piece with eyes goes straight into the ground.
The shared name misleads, because the two plants aren’t related. Sweet potatoes belong to the morning glory family, while potatoes belong to the nightshade family with tomatoes. Both store starch underground, so people long ago gave them similar names.
Yams, Onions and Other Underground Foods
- Potato: a stem tuber, a swollen underground stem.18Carnegie Museum of Natural History. Potatoes, Sweet Potatoes, and Yams: What’s the Difference?
- Yam: also a stem tuber, despite often being confused with sweet potato.19Carnegie Museum of Natural History. Potatoes, Sweet Potatoes, and Yams: What’s the Difference?
- Sweet potato: a root tuber, a swollen root.20Carnegie Museum of Natural History. Potatoes, Sweet Potatoes, and Yams: What’s the Difference?
- Carrot and turnip: enlarged taproots.21Food Unpacked. Root vegetables and tubers
- Onion: a bulb, a short stem wrapped in fleshy leaf bases.
The yam mix-up is common in the United States, where orange-fleshed sweet potatoes are often sold with a label saying yam. True yams are a different plant, usually sold with rough, bark-like skin and drier, starchier flesh. A true yam is a stem like a potato, while the sweet potato sold as a yam is a root.
You can sort most underground foods yourself. Eyes or buds across the surface point to a stem. Rings of leaf layers when you cut it open point to a bulb. A single tapering body with fine hair roots points to a taproot.
How the Plant Decides to Grow Potatoes
The leaves make a mobile protein signal and send it underground to the stolons.22PubMed Central. Regulation of storage organ formation by long-distance tuberigen signals in potato Its production is strongly linked with short days, which trigger potato formation.23PubMed Central. Regulation of storage organ formation by long-distance tuberigen signals in potato Plants that make extra of this signal form potatoes even under long summer days.24PubMed Central. The Multiple Signals That Control Tuber Formation Other signals from the leaves boost the message once it reaches the stolons and can raise yields.25PubMed Central. The Multiple Signals That Control Tuber Formation
Hormones then switch the stem from growing to storing. High levels of the growth hormone gibberellin delay and reduce potato formation.26PubMed Central. Regulation of storage organ formation by long-distance tuberigen signals in potato The arriving signal lowers gibberellin at the stolon tip, letting it stop stretching and start swelling.27PubMed Central. Regulation of storage organ formation by long-distance tuberigen signals in potato Sugar made in the leaves then flows down to the growing potatoes through the plant’s transport tissue.28PubMed Central. The Sucrose Transporter StSUT1 Localizes to Sieve Elements in Potato Tuber Phloem and Influences Tuber Physiology and Development Inside, the sugar is stored as starch grains packed in special storage cells.29PubMed Central. Starch metabolism in potato Solanum tuberosum L.
In a garden, this means timing and leaves matter. Many varieties start forming potatoes as days shorten in late summer or around the time the plant begins to flower, though timing varies by variety. Healthy leaves matter because they make both the swelling signal and the sugar that gets stored. If frost or disease badly damages the foliage early on, the plant has less sugar to send down, so the potatoes end up fewer and smaller. Letting the plants keep their leaves until they die back naturally lets the potatoes finish filling.
Why Potatoes Grow From Stems at All
Tubers gave the ancestor of the potato an edge in the cold, dry high-altitude Andes.30Natural History Museum. Scientists reveal the origins of the potato and find the key to unlocking climate-resilient varieties They let the plant reproduce without seeds or pollination and spread into new places.31Natural History Museum. Scientists reveal the origins of the potato and find the key to unlocking climate-resilient varieties Wild potato relatives all share tubers, a trait that arose from an ancient cross between a tomato ancestor and another plant group about 8 to 9 million years ago.32Cell. Ancient hybridization underlies tuberization and radiation of the potato lineage
A stem suits this job because it carries buds, so a stored piece of stem can grow into a whole new plant without seeds. Every potato from one plant is a genetic copy of it, which is why farmers plant potato pieces: it keeps a variety’s traits steady.