GROWTH CYCLES. “The growth of many trees is not continuous, but is interrupted by episodic rest phases. Pedunculate oak (Quercus robur L.), a major forest tree species in European forests, displays rhythmic growth with successive growth/rest phases of fixed periods of 18–22 d under constant culture conditions, which reflects the endogenic character of this trait.”
http://jxb.oxfordjournals.org/content/66/22/7113.full