1. Why the Head of a Horsetail Shoot Is Divided into Hexagons
If you look at the head of a horsetail shoot with a magnifying glass, you will see it is divided into hexagonal sections. These sections are the sporangia, and from the gaps between them, you can see something that looks like green powder [2]. Because the head resembles the tip of a brush, it is written with kanji characters meaning "earth brush." The name "horsetail" (sugina) is said to come from its appearance, which resembles the leaves of a cedar tree [2]. If the head of a shoot has opened completely, it is explained that the powder inside has already flown away [2]. So, what exactly is this powder?
2. Why the Powder from a Horsetail Shoot Is Spores, Not Seeds
This powder consists of spores. The horsetail is a fern in the Equisetaceae family, and ferns do not produce flowers or seeds. Instead, they increase their numbers using spores [3]. This is a different path from plants that reproduce by flowering and making seeds. If you consider the shoot as a stem created to produce and send out these spores, the meaning of the hexagons packed tightly in the head becomes clear. By the way, spores are light, like powder. If they simply fall at the base of the parent plant without being carried by the wind, they cannot spread far. Is there some mechanism to help them?
Perhaps the spores themselves have a mechanism that makes them easier to fly.
3. How the Threads on Horsetail Spores Move with Humidity
Spores have thin, string-like parts called elaters, which stretch and shrink according to humidity [4]. They shrink when there is moisture and spread out when it is dry [2]. The explanation in the sources is that by spreading out on a dry, sunny day, they become easier to catch on the wind [2]. When the air is humid, they curl up; when it is dry, they spread. If you want to disperse spores, a dry, sunny day seems more suitable. We will not count how many elaters each spore has here.
4. How a Fallen Horsetail Spore Becomes a Small Prothallus
A spore that rides the wind and falls to the ground does not immediately become a horsetail. First, it becomes a small body called a prothallus. The prothallus has male and female organs, and fertilization occurs there for the first time. After that, the body of the next horsetail grows [3]. The spore-bearing shoot, having finished releasing its spores, ends its role and withers [3]. However, the horsetail appears in the same place the year after it withered. Why does it come out every year even if the shoot dies?
If the withered shoot were the only thing to rely on, this would not make sense.
5. How the Underground Stem Connects Horsetail Shoots and Green Horsetails
It is explained that the shoot and the green stem of the horsetail are often connected by stems inside the soil [1]. This happens in many cases, but the sources do not say that every shoot is necessarily connected to the same clump. The sources explain that in spring, the shoot appears first. Around the time it releases spores, the green stems and leaves of the horsetail begin to grow [1]. The horsetail is a perennial fern that reproduces using both spores and underground stems [2]. It can be read that the reason it appears every year even after the shoot dies is that the underground stems continue to live. Although they look like different weeds, the shoot and the horsetail are two different forms of the same plant, showing different roles in each season.
6. Why Horsetail Is Hard to Control in Fields
The underground stems of the horsetail can extend long and sideways beneath the ground. Even if the above-ground parts are removed, if stems remain underground, new shoots may emerge from there. Because the underground stems can sometimes increase even when cut apart, they are often treated as a difficult weed to handle in fields. Hidden beneath the cute shoot that peeks out in early spring is this strong system. On the other hand, living underground stems help it return each year.
7. Where You Can See Fossils of Carboniferous Sphenopsids
Materials from the Gunma Prefectural Museum of Natural History feature actual fossils of sphenopsids from the Carboniferous period of the Paleozoic era [5]. The same materials explain that the genus Equisetum is the only genus within the class Equisetopsida that is still living today [5]. However, from this material, one cannot say that the current horsetail is the same as the plants from the Carboniferous period, or that it is a direct descendant of sphenopsids.
8. What You Can Investigate Next About Horsetail Shoots
Comparing the state of shoot heads is one way to investigate. If the head is closed, the spores are still inside. If it is fully open, the spores have already flown away [2]. If you compare several in the same place, you might see differences in how far the dispersal has progressed. The speed at which shoots grow and the photosynthesis of the green horsetail stems are questions not covered in this article. There is room for you to check these yourself by looking at encyclopedias or museum exhibits.