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."
In my opinion, the seed discussion is a metaphor for the cycle of life. On the first page of chapter 8, Pip notices that Pumblechook is a seedsman and Pip considers the trade wondering if those seeds ever want to break the limits of their bags and drawers and just bloom.
We see Pip want to do this throughout the book. He wants to break the boundaries that he has to stay within in order to receive his money from his benefactor or to keep receiving education.
Then Pip notices that Pumblechook is always eyeing the saddler with great anxiousness, and the saddler the next profession and so on. Each of these men can look to the next for the clue as to if their business will soon flourish. Each is next in the cycle of business. It's like if the shoe cobbler is making lots of shoes, the leather maker can get excited. If the leather maker is getting lots of business then the skinners of hides can get excited.
The seed represents growth, not just the growth of a flower, but of a man, and a community.