Even after the general acceptance of Monocots and Dicots as the primary groups of flowering plants, botanists did not always agree upon the placement of families into one or the other class. Even in this century some plants called paleoherbs have left problems for taxonomy of angiosperms. These plants have a mix of characters which do not occur together in most other flowering plants. For instance, the Nymphaeales, or water lilies, have reticulate venation in their leaves, and what may be a single cotyledon in the embryo. It is not clear whether it is a single lobed cotyledon, or two which have been fused. The water lilies also have a vascular arrangement in their stem similar to that of monocots.