The Sitka data frame has 395 rows and 4 columns.  It gives repeated
  measurements on the log-size of 79 Sitka spruce trees, 54 of which
  were grown in ozone-enriched chambers and 25 were controls.  The size
  was measured five times in 1988, at roughly monthly intervals.
SitkaThis data frame contains the following columns:
sizemeasured size (height times diameter squared) of tree, on log scale.
Timetime of measurement in days since 1 January 1988.
treenumber of tree.
treateither "ozone" for an ozone-enriched
      chamber or "control".
Venables, W. N. and Ripley, B. D. (2002) Modern Applied Statistics with S. Fourth edition. Springer.