TOLEDO BEND RESERVOIR, Texas - One biologist compares the persistent green weed to "The Blob," the title character in the 1950s sci-fi classic flick that grows and grows and consumes everything in its path. Other scientists describe the plant as looking like little heads of lettuce or squished green grapes. Then they use terms like noxious, invasive and just plain scary. Even the species name sounds sinister: salvinia molesta. No one has anything good to say about what's more commonly known as giant salvinia, a Brazilian tropical floating fern that's found a home in slow-moving streams and freshwater ponds and...