Final answer:
Horsetails are characterized by their highly flammable spores, jointed stems with whorled branches and vestigial leaves, and multi-flagellated sperm for reproduction. They have adapted to live in a variety of moist environments but are not strictly aquatic.
Step-by-step explanation:
Characteristics true of Horsetails include highly flammable spores. Horsetails, or Equisetum, are distinctive non-seed vascular plants with a dominance of the sporophyte generation and a small gametophyte generation. Horsetails possess perennial rhizomes, vertical branches with a unique growth pattern, photosynthetic stems, and very small, scale-like leaves. The spores of horsetails are produced in cone-shaped structures and are disseminated by wind, aided by elators, which respond to moisture changes. These spores can ignite easily, making them highly flammable. Horsetails have joints or nodes in their stems, hence the former name Arthrophyta, from which whorled branches and vestigial leaves emerge. Photosynthesis primarily occurs in the green stems rather than in the needle-shaped leaves.
The sperm of horsetails are multi-flagellated and require a medium such as water to reach the egg for fertilization. This rules out the choice of sperm with a single flagella. Although horsetails can be found in moist environments, they are not exclusively aquatic, which eliminates the aquatic characteristic.