White Mustard Seeds : This relates to the Buddha‘s response to a woman who came to him distraught at the loss of her child. He instructed her to collect a mustard seed (as common as salt or pepper at the time) from every home that never had witnessed a death. She returned empty-handed. The Buddha showed her that she was not alone in her sorrow and that death is an inescapable part of life. Mustard seeds represent Right Understanding.
My Prof pointed out something interesting to me: He said, you know all those religious ceremonies we perform, that appear to be nonsense to people outside their faith. Those ceremonies may not actually mean anything, but they lead to meaning something. In this case, collecting mustard seeds meant nothing, but the process of collecting seeds and realizing that ‘she is not alone’ was what was significant.