Epilobium Hirsutum and E. Parviflorum

These species of Epilobium (Onagraceae) are native to Great Britain and normally flower in summer, and it seemed likely that their flowering — if determined at all by photoperiod — would be induced by LD. However, no references were found in the literature referring to the physiology of their flowering. In the case of E. hirsutum, several references to experimental work on phyllotaxis ¹ and some papers on genetic aspects were noted. However, one interesting mention of E. hirsutum with regard to its vegetative morphology was found in Klebs’ book, ² in which he noted that in this species horizontal shoots are produced during winter. It seemed probable that such winter shoots would be controlled by photoperiod rather than temperature, or perhaps by interaction of both environmental factors, and this was confirmed subsequently by experiment. In summer, E. hirsutum produces tall flowering shoots some 90 to 150 cm in length, while E. parviflorum is rather smaller, reaching only 30 to 60 cm.