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[?].