Anisocotyly, the unequal development of cotyledons post germination, is a unique trait observed only in Old World Gesneriaceae (Lamiales). New World Gesneriaceae have isocotylous seedlings. In both Old and New World Gesneriaceae, cotyledons initially grow equally for a short period just after germination. In the New World species, both cotyledons cease their growth at the same time early on, whereas in Old World species one cotyledon continues to expand to become a macrocotyledon while the other withers away. In this study, cotyledon growth was observed in two European Old World Gesneriaceae: Haberlea rhodopensis and Ramonda myconi. The results were compared with those for the typical anisocotylous species Streptocarpus rexii and the typical isocotylous species Corytoplectus speciosus. We found that the cotyledon growth patterns in Haberlea rhodopensis and Ramonda myconi were intermediate between the typical anisocotylous or isocotylous species. Haberlea rhodopensis and Ramonda myconi showed irregular growth patterns, with some plants being slightly anisocotylous but most being isocotylous. The developmental basis for the residual anisocotyly, the extended basal meristem activity in the macrocotyledon, appeared to be identical in the European species to that in the typical Old World Streptocarpus rexii but weakly expressed, rare and terminated early. In conclusion, European Gesneriaceae retain a reduced anisocotylous growth that may be linked to their early plumule development.

Anisocotyly, the prolonged meristematic growth of one of the two cotyledons, is a distinctive feature of Gesneriaceae subfam. Cyrtandroideae. The larger cotyledon, the macrocotyledon, often grows to resemble a normal foliage leaf, and in some taxa may be the only foliar organ of the plant. This raises a number of questions. Which cotyledon becomes the macrocotyledon? Is this pre-determined in the embryo or differentiated after germination? Which external factors such as gravity and light are involved? The observation that the macrocotyledons of unifoliate Gesneriaceae growing on steep rocks mostly point downwards suggests that gravity is involved, but it is not clear whether it plays an initial determinant role or merely later influences orientation. In order to identify possible controlling factors, several experiments were performed, mostly on material of Chirita lavandulacea and Streptocarpus rexii. All seedlings responded significantly to light, the cotyledon further from the light source becoming the macrocotyledon. Seedlings growing in inclined pots with the light source below the pots mostly developed an upper macrocotyledon. The explanation proposed is that this cotyledon receives more light when the two cotyledons unfold after germination. Later on, apparently due to gravity, Chirita seedlings showed re-orientation with the macrocotyledons ultimately pointing downwards, though in Streptocarpus no such downwards re-orientation was observed. This difference is probably correlated to a difference in hypocotyl structure. Our conclusion is that while light is the initial factor controlling macrocotyledon development, gravity may cause re-orientation in some species.