More than 900 plant species are known to be able to internally heat themselves. A famous example is the Skunk Cabbage, Symplocarpus foetidus, which is capable of maintaining an internal temperature of around 20 °C even when the ambient temperature drops as low as 0 °C.

Other examples : Cycads, Water lilies, Palms, Custard apple, etc etc.

Because flowers lack the complex nervous system seen in mammals, the maintenance of a stable flower temperature in floral tissues must be attained by a defined thermoregulatory mechanism that responds to the changes in the environmental air temperature"

See: Structural requirements for the perception of ambient temperature signals in homeothermic heat production of skunk cabbage (Symlocarpus foetidus) Plant, Cell & Environment, Volume 26, Issue 6

The mechanisms have yet to be fully explained, although it's currently thought likely that the heat originates in cellular mitochondria.

One hypothesis for the evolutionary existence of the phenomenon is that it may help disperse scents that could attract pollinating insects.