Part I of this work on Growth Periodicity (Proc. Roy. Soc. Edin., vol. xxxiii, Part I (No. 8), p. 85) dealt with the occurrence of a four-day periodicity in plant organs and with rhythm in roots. With reference to the latter, the following conclusions were deduced from various observations made up to that time:—

1. “Roots exhibit a periodicity under ordinary conditions of environment which differs from that of shoots.

2. “Owing to correlation, the root periodicity is affected by changes in the root rhythm, but to what extent has yet to be determined.”

Since then, numerous experiments have been performed to find out whether any correlation exists between the growth of the root and the shoot, and if so, the nature of such a correlation.