Viroids are infectious agents that consist of single-stranded
RNA. They are much smaller and simpler than
viruses and lack the
protein cover that is typical for viruses. Viroids use higher
plants (such as
potatoes,
tomatoes and
cucumbers) to reproduce, inserting themselves into the
nucleus of a plant cell to be replicated there. Viroids are usually transmitted by
seed or
pollen. Infected plants can show distorted growth. The first viroid to be identified was the
potato spindle tuber viroid[?].