And that stripped, athletic leg, hairy, In a fling of abandon –"
This is the first reference we get to anything that's not the top part of the flower. Additionally, it's also the last part of the list that started a few lines ago with "we could see nothing but" – so now the whole plant has been included in this kind of singular world-view.
We're talking now, of course, about the stem, which on a poppy is very long, has no leaves (all the leaves are right near the base, almost on the ground), and is covered in a kind of rough fuzz. (See a picture here.)
That's kind of convenient, really, when you've been comparing everything to body parts – that stem, long and singular as it is, could be said to be almost exactly like a leg.
Why "athletic"? Well, if you look at a poppy, it seems nearly unbelievable that a stem that long and comparably thin could hold up such a huge blossom, but it does. As far as plants are concerned, we'll go ahead and say that that's pretty athletic.
As a closing line, "in a fling of abandon" pretty well sums up the speaker's attitude towards the life cycle of this flower. After all, a "fling" is something that only lasts a short while, and in this case, "abandon" doesn't necessarily mean to leave something behind so much as it means "without thinking."
The poppy's existence, in this poem, has been a kind of exuberant explosion – not so big on the forethought, but huge on the intensity of expression. It's poof! COLOR! then gone, and we're left reeling from the force of it. Phew. No ordinary flower, as it turns out.