1. The Holes in a Lotus Root Are Air Pipes Running Up to the Leaves
Short answer: the holes carry air. A lotus root is not really a root. It is a thick, swollen part of an underground stem (a rhizome) that grows sideways through the mud at the bottom of a pond [1]. Mud holds very little oxygen, and a living stem still needs to breathe [2]. The lotus helps deal with this by moving air down from its leaves through the holes. Explanations written for the public say the air passages continue up the long leaf stalk (petiole) and open into the leaf above the water [2].
You may have heard that a lotus root always has ten holes, one in the middle and nine around it. A Japanese food guide says that is not so: many lotus roots do not have exactly ten, and the number and layout differ from one root to the next [1]. If the number is not fixed, the holes look less like a pattern and more like a tool. The next sections ask how the air gets in, and why it is more complicated than a simple pipe.
2. Why a Stem Buried in Mud Needs Air Delivered From Above
Plants that live with their roots and stems underwater or in mud face a problem: oxygen moves very slowly through water, and mud uses up what little there is. The lotus rhizome is alive, so it uses oxygen and gives off carbon dioxide, just as we do. Public explanations say that air taken in through the leaves is carried through the holes to the rhizome [2].
A simple way to picture it is a snorkel: a tube from your mouth up to the air above the water. In the lotus, the holes in the rhizome and the passages in the leaf stalk work like the tube. The picture has one big limit, though. A person using a snorkel breathes the air in through the tube. A lotus has no lungs, so something else must move its air. That is the puzzle for the next section.
3. How the Leaf Pushes Air Down Into the Holes
Researcher John Dacey measured the gas inside lotus leaves and reported in 1987 that the pressure in the leaf's gas spaces is higher than the air around it. The upper surface of a lotus leaf has two kinds of regions. In Dacey's measurements, across most of its area (more than 99%, by the paper's account), gas can get through only very tiny pores, narrower than 0.03 micrometers on average, which is far thinner than a hair. A central patch of the leaf, directly opposite where the stalk joins, has much bigger pores. Under his measurement conditions, air entered across the wide leaf surface and left through that central patch [3].
So in those measurements the leaf acted like an entrance with a separate exit. Researchers later described the system in more detail: a network of gas canals runs from the leaf blade through the leaf stalks and into the rhizome. At the leaf center there are unusually large pores (stomata), sitting over a junction that links most of the canals in the blade and the larger pair of canals in the stalk [4]. (I read Dacey's abstract and a summary of his paper, not the whole paper.)
4. Does the Air in a Lotus Always Flow the Same Way?
A common explanation says air enters through young leaves, travels down their stalks, passes through the rhizome, and leaves through older leaves, carrying carbon dioxide made by the rhizome [2]. That is easy to remember, like a loop of water through a pond. But careful measurements show that the real flow is less simple.
In 2014, Philip Matthews and Roger Seymour of the University of Adelaide, Australia, reported that the big stomata at the leaf center open in the morning, close around midday, and open again in the afternoon. They found that the midday closing happens at the same time as a temporary reversal of the air direction in the leaf-stalk canals. Their experiments also showed that these stomata let less air through as light gets brighter [4]. Another summary from the U.S. Geological Survey says lotus leaf stalks have separate canals with air moving in two directions, unlike water lilies, whose flow is one-way [5].
Why would a plant reverse its airflow? The authors suggest the lotus may use its stomata to control the pressure, direction and speed of the air, which may ventilate the rhizome and perhaps guide carbon dioxide from the mud toward the leaves, where it can be used for photosynthesis [4]. This is described as a hypothesis, not a proven answer. The sources I read also do not say how far a single hole runs through a rhizome from one segment to the next.
5. Why Lotus Root Is Served at Celebrations in Japan
In Japan, lotus root is a popular dish at celebrations because the holes let you see through to the other side. The saying is that it gives a "good view of what lies ahead." That is the meaning given by the Niigata Prefecture food guide [6]. It is a play on words and a tradition, not biology.
It is interesting how the two ideas meet, though. In the saying, the holes let you see ahead. In the plant, the holes let air pass from the mud all the way to the leaves. Both are about a way through. The real passage turns out to be busier than a straight line, with air that can slow down, change direction and speed up through the day.
6. How to Look at a Lotus Root's Holes Yourself
If you have a lotus root from a grocery store, ask an adult to cut it while you watch from a safe distance, and look at the cut surface. Count the holes. Is it ten? Are the big holes the same size, and is there a tiny one in the middle? Have the adult cut another piece from a different part of the same root and compare. Notice, too, whether the holes line up when you look through a long piece held to the light. For deeper reading, the Matthews and Seymour paper and its short summary on botany.one are good starting points [4].