According to Lowell's theory, what were the visible dark lines on Mars, rather than the water channels themselves?

Answer

seasonal greening along the banks

Lowell posited that the visible lines were not the water channels, which he felt would be too wide to see, but rather the cultivation or vegetation along those banks that changed seasonally.

According to Lowell's theory, what were the visible dark lines on Mars, rather than the water channels themselves?
MarsastronomyvegetationCanalsLowell