While the Whale is falling it tells you what questions he is asking himself like what is my life worth or other questions about his life.The whale is also one of my themes underneath meaningless because "what is the meaning of this whale being here.The potted plant was one of the missiles that was hunting down the Heart of Gold but due to the improbability drive it changed into a potted plant of lovely flowers.
The main reason I think that these are symbols is because when you read this bit in the book or watch it in the movie it is something that catches your attention and make you want to engage in what is happening while you read each word.