The freshwater macrophyte Hippuris vulgaris exhibits heterophylly. During July and August aerial-type leaves occur even on immersed shoots down to a depth of 15 m in two clear-water limestone lochs in Scotland. Complementary field and laboratory data indicate that a high intensity photoperiod and temperatures above 10°C are prerequisites for aerial-type leaf production on submerged shoots. In these conditions, regular injections of additional sucrose, the natural photosynthate, at 05 mol kg−1 fresh weight will not induce the aerial-type leaf. There is relatively little attenuation of blue light with depth in these lakes and additional blue light just after the photoperiod does not induce the aerial-type leaf. There is no evidence for an ontogenetic or circadian photoperiodic requirement for the production of aerial-type leaves, or that heterophylly is mediated by ethylene.

However, a low ratio of red (R 660 run) to far red (FR 730 run) light throughout, or very dimly for a brief period just after, the photoperiod treatment regularly induces aerial leaf formation on submerged shoots of any length. A similar low R/FR ratio is found naturally in these lakes in summer at depths where submerged aerial leaves occur. Selective attenuation of far red by lakewaters raises the R/FR ratio steeply with increasing depth. At R/FR ratios above a critical range, which is similar in field and laboratory, aerial leaf formation is inhibited and only submerged-type leaves are formed. Simply by changing the R/FR ratio, either during or briefly after the photoperiod, transitions between submerged and aerial-type leaves are repeatedly reversible, implying direct phytochrome control.