The plastochron and leaf plastochron indices are methods created by R.O. Erickson and F.J. Michelini in 1957 to measure the age of whole plants and individual leaves based on their morphology rather than chronological age. They have been particularly useful for detailed studies of whole shoot and leaf development. Since the indices are based on measurements of exponential plant growth, they provide an opportunity to demonstrate the value of applying mathematics to a biological problem. I will describe a laboratory exercise from a upper-division Plant Structure course in which students are presented with the theoretical basis of the indices and then calculate them for Xanthium strumarium (cocklebur) plants of various ages (4-6 weeks). The exercise demonstrates both the potential variability in plant growth and the results of an exponential growth rate. Pre- and post-tests showed a significant increase in the studentís understanding of exponential growth and their ability of work with logarithms. This exercise was created as part of the BioMathLab project at USU supported by Department of Education FIPSE grant P116B 71688.

Key words: Xanthium strumarium, exponential growth, plastochron index, shoot development