Jean Toomer's "Reapers" possesses a dark, gloomy, and pessimistic tone. (Tone illustrates the author's attitude toward the subject.) In the case of this poem, the speaker openly illustrates his or her anger with a mower's inability to differentiate between a life (a rat) and a crop. Humans who reap a field are able to stop and contemplate their harvesting. Mowers, very unlike humans, do not stop for anything. Instead, they simply plow over everything (illustrated by the "blood-stained" blades).
The speaker's tone illustrates that he or she is worried about what is to come if machines are allowed to continue their thoughtless mowing. It seems that the speaker assumes, and fears, that machines will continue to push forward without consideration of what is in front of it. Essentially, the speaker seems to believe that the machines will be the end of the world (something which proves to be dark, gloomy, and utterly pessimistic).