I am no expert on the language of flowers, sometimes called floriography, but its obvious Rossetti dd and often added extra meaning to his paintings, obvious to his audience but hard for us now.
Here he shows Jane sitting in the branches of a sycamore tree and holding a sprig of honeysuckle. This sweet-smelling climbing plant symbolised the bonds of love for the Victorians, and Rossetti may have included it here as a subtle reference to the relationship between artist and model. Rossetti was also a poet, and the title relates to his poem of the same name which ends:
She dreams; till now on her forgotten book
Drops the forgotten blossom from her hand.
For other flowers (and colours)