In botany the plus sign is used to symbolize that a plant is a chimera, containing two or more genetically different cell strains or tissues.
This symbol is not used for animal chimeras for the obvious reason that chimeras are considerably more common, and reproducible, in the field of botany than the field of zoology. While plants may gain a plus symbol through spontaneously mutated cells, the most common use is the result of grafting (the result of which may be specifically called a graft chimera).
There are two ways to write the name of chimeras: the most obvious is by listing the names of both parent genera in alphabetical order, joined by the plus sign: Crataegus + Mespilus. Another option is to combine the two names, in which case the plus sign is added to the beginning of the portmanteau name: +Crataegomespilus.
Because a chimera is due to the presence of two kinds of plant tissue, it must be propagated by a vegetative method to preserve both types of tissue in relation to each other. As such, technically a chimera should not be given a full scientific name, and should always lack the species name. This convention is not always practiced.
You will often also see X used in the same way to denote a crossbreed.