They are the so-called "canals" discovered by schiaparelli in 1877. The term "canal" is an unfortunate one.
"Lectures in Navigation"
Ernest Gallaudet Draper
The distinguished Italian astronomer, schiaparelli, who has studied these formations longer than any one else, seems inclined to regard them as water courses lined on either side by vegetation, which flourishes as far back from the central channel as water can be supplied from it-a plausible enough explanation if the fundamental difficulty about temperature can be overcome.
"A Text-Book of Astronomy"
George C. Comstock
A distinguished Italian astronomer, Professor schiaparelli, some years ago announced a remarkable discovery with respect to the rotation of the planet Mercury.
"The Story of the Heavens"
Robert Stawell Ball