By the road to the contagious hospital (line 1)
This is pretty much the first and the last time we’ll hear about man-made things in this poem. All the rest of it is about nature. So, why is this an important theme and not just a quick reference? Well, the fact that it comes first is a big clue. Williams uses this line to set up different two worlds. On the one hand, we have nature, with its fields, trees, grass, etc. On the other hand, we have the world of hospitals and roads, and, therefore, also the world of jobs, cars, responsibilities, etc.
All along the road (line 9)
Oops, we lied. Here’s that road again. Do you feel the way it cuts back across the poem? We’re meant to notice the difference between the natural shapes of the plants and the idea of a man-made road laid across the landscape.
They enter the new world naked, cold, uncertain of all (line 16-17)
Human objects are gone from the poem, and the speaker doesn’t talk about him or herself at all. There’s no description of how this makes the speaker feel, what he or she thinks – nothing. So. why is this still a poem about Man and the Natural World? One big clue is the descriptive techniques the poem uses. Here, the poem gives plants human feelings. Williams tells us that they feel cold and uncertain, that they are naked. Plants don’t feel anything, as far as we know. Carrots don’t worry about being naked. All of these are human (or, at least, animal) feelings. Williams brings man back into the picture in a subtle way. It turns out that man and nature aren’t so easy to pull apart in this poem.