ME. FRANCIS DARWIN has made an interesting addition to his important discovery of nectar-bearing glands on the young fronds of Pteris aquilina, supplied from the ever-welcome experience of Mr. Fritz Muller. The latter gentleman finds that in Brazil the Pteris aquilina is protected from the leaf-cutting ants by those attracted to the nectar, and Mr. Darwin adds some speculations on the origin of the glands and their continued functional activity in Europe where they now appear to be useless. On this part of the question I should like to make the following remarks:—