Their phylogenetic origin is vague, tough it has always been considered that viruses are genes that became vagrant after having excluded themselves of the host s or a related species genome. During the last years, an alternative explanation assuming viruses to be by-products of RNA processing has been discussed. All suggestions remain hypotheses, though, since no definite proof for either of them exists yet. Viroids consist only of RNA, i.e. they contain no protein at all. Except for a few cases, viruses are not surrounded by a membrane.
Viruses have no energy metabolism of their own.
Consequently, they cannot perform syntheses and are thus unable to replicate themselves. Instead, plant viruses can be grouped in a number of varieties . The name of a virus is usually derived from the name of its main host plant. Although with viruses, the term species may not quite correspond to the way it is defined in biological systematics, it is perfectly reasonable and common to use it for viruses, too, since all viruses and viroids contain an original genome with a species-specific information. Its continuity over generations is guaranteed by replication in the host cells.