Which plant develops dense mats of slender rhizomes and has many roots?

The plant that develops dense mats of slender rhizomes and has many roots is Hydrilla (Hydrilla verticillata). Hydrilla is known for its vigorous growth and ability to form dense underwater beds, which can significantly alter aquatic ecosystems. Its slender rhizomes allow it to spread horizontally, leading to the formation of extensive mats in water bodies. The dense root system contributes to its ability to anchor itself firmly in the substrate and compete effectively for resources such as nutrients and sunlight.

In contrast, the other options do not exhibit the same extensive growth characteristics. Leafy Pondweed, Sago Pondweed, and Richardson's Pondweed, while having their own unique adaptations and growth patterns, do not develop the same dense mats or vigorous rhizome systems as Hydrilla. Understanding the growth habits of these aquatic plants is crucial in managing aquatic environments, especially when dealing with invasive species like Hydrilla.