The Mushroom is the Elf of Plants — Upstart

Mushroom has the definition of "a person or family that has suddenly sprung into notice; an upstart (OED 2a)."

The Mushroom is the Elf of Plants-1
At Evening, it is not-
At Morning, in a Truffled Hut
It stop upon a Spot

As if it tarried always 5
And yet its whole Career
Is shorter than a Snake's Delay
And fleeter than a Tare-

'Tis Vegetation's Juggler-9
The Germ of Alibi-
Doth like a Bubble antedate
And like a Bubble, hie-

I feel as if the Grass was pleased 13
To have it intermit-
This surreptitious scion
Of Summer's circumspect.

Had Nature any supple Face 17
Or could she one contemn-
Had Nature an Apostate-
That Mushroom-it is Him!
(F.1350/J.1298)
[1] Mushroom:: a person or family that has suddenly sprung into notice; an upstart (OED 2a). Plants:: the harvest.
[2] Evening:: the condition of being equal.
[3] Morning:: the beginning. Truffled Hut:: a poor living condition.
[4] Spot:: a small place.
[5-8] whole Career, fleeter:: the upstart's sudden rise.
[9] Vegetation:: dull, empty, or stagnant life spent in retirement (OED 4)
[10] Germ of Alibi:: the origin of alibi, as an upstart denied his living in the hut.
[13] the Grass:: idling people. Grass:: being free from fixed engagements (OED 5b).
[14] intermit:: to interpose, put between (OED v.2 2); an upstart is welcomed by the idling people.
[15] surreptitious:: the stealthily rise of an upstart. Scion:: a graft, a poor scion to a rich tree.
[16] Summer:: one who sums or adds; an upstart is cautious in making friends like doing math.
[17] supple Face:: a face can be changed easily.
[19] Nature:: human nature.
[20] Him:: a hint on a man, not a plant.

For being supposed to be a disease peculiar to aristocracy, every upstart is fain to affect it. You cannot please a mushroom squire, or a retired shop-keeper better, than by telling him his disease is "gout" ─ Fallacies of the Faculty (1841)