The Spruce data frame has 1027 rows and 4 columns.
This data frame contains the following columns:
a factor giving a unique identifier for each tree.
a numeric vector giving the number of days since the beginning of the experiment.
a numeric vector giving the logarithm of an estimate of the volume of the tree trunk.
a factor identifying the plot in which the tree was grown.
Diggle, Liang, and Zeger (1994, Example 1.3, page 5) describe data on the growth of spruce trees that have been exposed to an ozone-rich atmosphere or to a normal atmosphere.