When Pip goes to stay with Uncle Pumblechook on High Street in London, he writes,
...and I wondered when I peeped into one or two on the lower tiers, and saw the tied-up brown paper packets inside, whether the flower-seeds and bulbs wanted [needed] of a fine day to break out of those jails and bloom
In an extension of this metaphor of the seeds being jailed, Pip himself feels sympathy with them as he is tightly confined in a little bed in the cofining attic with the sloping roof. This compartmentalizing of Pip then extends to Uncle Pumblechook and the other merchants. Albeit a sycophant of the aristocracy, Uncle Pumblechook is yet, like the others of the middle class, restricted to his own level of society.
The extended metaphor of the confinement of the seeds as though they are in jail imitates the societal jail of the merchant class and of other classes in Charles Dickens's Great Expectations:
I discovered a singular affinity between seeds and corduroys (that Pumblechook wears)....there was a general air about the corduroy, so much in the nature of the seeds, and a general air and nature about the seeds, so much in the nature of the corduroys...that I hardly knew which was which.
Clearly, then, Dickens's metaphor of the seeds reminds readers of the motif of society as a prison in which no one can truly rise above or escape one's class. Try as he may, Uncle Pumblechook who aspires to become an aristocrat, can only remain in his "corduroys" with the seeds that share the "general air."