The Data in the Figure Show a Relationship Between the Mean

The data in the figure show a relationship between the mean flowering date for many plant species around Concord, New Hampshire, and the mean spring temperature over a time span exceeding 150 years. However, there is a lot of scatter around the best-fit line in panel b. What information in the chapter might suggest the source of this scatter, and what does it imply about the plant species' cues for flowering?