The primary method by which horsetail spores are dispersed is through wind.

Explanation:
Horsetails are primitive plants that reproduce by spores rather than seeds. The spores are small and lightweight, allowing them to be easily carried by the wind to new locations. This method of dispersal is known as anemochory.

Advantages of wind dispersal:
1. Long-distance dispersal: Wind can carry spores over long distances, allowing horsetails to colonize new areas and expand their range.
2. Increased genetic diversity: Wind dispersal helps to mix the genetic material of different individuals, promoting genetic diversity within horsetail populations.
3. Exploitation of different habitats: Wind dispersal enables horsetails to reach and colonize a wide range of habitats, including open fields, forests, and wetlands.

Adaptations for wind dispersal:
1. Lightweight spores: The spores of horsetails are small and lightweight, facilitating their transport by wind currents.
2. Elongated structures: Horsetails have elongated structures called sporangiophores that hold the spores. These structures are arranged in clusters called strobili, which are positioned at the top of the plant for optimal wind exposure.
3. Spore release mechanisms: Horsetails have specialized mechanisms for releasing spores. When the spores are mature, the strobili open up, allowing the wind to disperse the spores.

Other methods of spore dispersal:
While wind dispersal is the primary method for horsetail spores, they can also be dispersed by water or animals to a lesser extent.
- Water: In wet habitats, such as marshes or along river banks, horsetail spores can be carried by water currents to new locations.
- Animals: Though less common, horsetail spores can sometimes hitch a ride on the fur or feathers of animals, allowing them to be transported to different areas.

Overall, wind dispersal is the most effective method for horsetail spores, enabling these primitive plants to colonize various habitats and ensure their survival and reproduction.