The Gunnerales are an order of flowering plants. In the APG III system (2009) it contains two genera: Gunnera and Myrothamnus. These are assigned to two separate families (Gunneraceae and Myrothamnaceae, respectively). In the Cronquist system (1981), the Gunneraceae order was placed in the Haloragales and Myrothamnaceae in the Hamamelidales. Although DNA analysis was definitive, the grouping of both families was a surprise, given their very dissimilar morphologies. In Cronquist's old system (1981, 1988), as well as Takhtajan's (1997), Gunneraceae actually belonged to the Rosidae, while Myrothamnaceae belonged to the Hamamelis. In modern classification systems such as APGIII, this order was the first to derive from the core eudicots.
Both families contain ellagic acid. Phloem cells contain a large number of plastids and the leaves have dented borders.
The plants are dioecious, have small flowers without perianth, and the stigma is at least weakly secretory. Gunnerales characters that share with the core of the eudicots are cyanogenesis via phenylalanine, metabolic pathways of isoleucine or valine, presence of the DNA sequence of PI-dB motif, 9 and is common to suffer a small deletion in the sequence of 18S ribosomal DNA. The characters which it shares with the core of eudicotiledóneas, and also with Buxales and Trochodendrales are: absence of alkaloids bencilisoquinolínicos, euAP3 + TM6 genes (gene duplication paleoAP3: Class B), and loss of mitochondrial gene rps2.
Typically a leaf is a thin, flattened organ borne above ground and specialized for photosynthesis, but many types of leaves are adapted in ways almost unrecognisable in those terms: some are not flat (for example many succulent leaves and conifers), some are not above ground (such as bulb scales), and some are without major photosynthetic function (consider for example cataphylls, spines, and cotyledons).
Conversely, many structures of non-vascular plants, or even of some lichens, which are not plants at all (in the sense of being members of the kingdom Plantae), do look and function much like leaves. Furthermore, several structures found in vascular plants look like leaves but are not actually leaves; they differ from leaves in their structures and origins. Examples include phyllodes, cladodes, and phylloclades.