I'm not sure specifically. It could be a personal story where Dennis has some kind of relationship with celery. Maybe Dennis grows celery, or there was an incident involving Dennis and celery that led to a long - winded tale.
There could be numerous interpretations. It might be a fictional story where Dennis is a character who has an emotional connection to celery. For example, he might have used celery to heal a sick animal in a made - up land, and the whole process of discovery, trial, and success made up the long story.