"The Solitary Reaper" is a poem that gives expression to the overwhelming influence that nature exercises on a romantic poet, especially William Wordsworth.
As the poet listens to the "plaintive numbers" he appears mesmerized. Nature, to Wordsworth, is more than natural scenes like forest, rivers and others. It includes songs coming from nature, common people, rural life and indeed anything that can be identified with nature.
In the poem, the valley is said to be overflowing with the song. Indeed, the song and the valley are fused together and both exercise a profound influence on the speaker. The influence is such that long after he leaves the scene he still carries the song in his heart.
The idea of the nightingale being able to sing "welcome notes" to "weary bands of travelers" or the 'cuckoo bird' singing "to break the silence of the seas" also shows the power of nature on man.
It needs to be noticed that the song's influence on the man is everlasting. It has a soothing effect on many unhappy encounters he has had in life.