"There's rosemary, that's for remembrance; pray, love, remember: and there is pansies. that's for thoughts." - Ophelia,Hamlet, Act 4, Scene 5.
The other flower mentioned besides rosemary in that sentence is pansies. I hadn't really heard much about the symbolism of pansies, but I think they're lovely, so I decided to do some research!
Pansies are a hybrid of several families of violas, and were bred and cultivated in the early nineteenth century. That means that really Ophelia couldn't have possibly had modern pansies (and neither could Shakespeare), though that name was used to refer to violas as well.
This embroidery was done by a young Princess Elizabeth (later Queen Elizabeth I) at age 11. The initials, KP, were that of her stepmother Katherine Parr, for whom the gift was made.
Pansies go by many names. I found this snippet from Wikipedia especially useful when looking for info on how to use the pansy in ritual or symbolism:
"The name “heart’s-ease” came from the woman St. Euphrasia, whose name in Greek signifies cheerfulness of mind. The woman, who refused marriage and took the veil, was considered a pattern of humility, hence the name “humble violet”.[ The specific colors of the flower – purple, yellow, and white – are meant to symbolize memories, loving thoughts and souvenirs, respectively."
There is also this explanation of where the name pansy comes from:
"The name 'pansy' is derived from the French word pensée meaning "thought", and was so named because the flower resembles a human face; in August it nods forward as if deep in thought."
Here is an image by Grandville from 1846 of a personified pansy, deep in thought:
In Shakespeare's A Midsummer Night's Dream the love potion used to create so much of the midsummer mayhem is made from pansy, here called by one of its other names, love-in-idleness (referring to the type of love where one wants to do nothing but be with one's beloved):
"Yet marked I where the bolt of Cupid fell.