Density‐dependent interactions between seedlings of Dactylorhiza majalis (Orchidaceae) in symbiotic in vitro culture

Growth of the heterotrophic seedlings of Dactylorhiza majalis (Rchb.f.) Hunt & Summerh. (Orchidaceae) in vitro with a symbiotic fungus (Rhizoctonia sp.) is density‐dependent, even at densities that are not high enough to exploit the growth medium fully. Competition was two‐sided; increasing density among seedlings did not increase size inequality between them. The slowly growing and smaller seedlings are normally more likely to die than the bigger ones, but mortality was not increased by higher density within the studied range. Growth depression by neighbouring seedlings is independent of their physical distance but varies with the total number of seedlings sharing the same culture dish and the same mycelium. Therefore, if the growth‐promoting effects of different fungi or treatments are to be compared in future work, the density of planting in the culture dishes should be comparable.