<P> There are three genera of Geminiviridae that possess geminate particles which are like two isometric particles stuck together . </P> <P> A very small number of plant viruses have, in addition to their coat proteins, a lipid envelope . This is derived from the plant cell membrane as the virus particle buds off from the cell . </P> <P> Viruses can be spread by direct transfer of sap by contact of a wounded plant with a healthy one . Such contact may occur during agricultural practices, as by damage caused by tools or hands, or naturally, as by an animal feeding on the plant . Generally TMV, potato viruses and cucumber mosaic viruses are transmitted via sap . </P> <P> Plant viruses need to be transmitted by a vector, most often insects such as leafhoppers . One class of viruses, the Rhabdoviridae, has been proposed to actually be insect viruses that have evolved to replicate in plants . The chosen insect vector of a plant virus will often be the determining factor in that virus's host range: it can only infect plants that the insect vector feeds upon . This was shown in part when the old world white fly made it to the United States, where it transferred many plant viruses into new hosts . Depending on the way they are transmitted, plant viruses are classified as non-persistent, semi-persistent and persistent . In non-persistent transmission, viruses become attached to the distal tip of the stylet of the insect and on the next plant it feeds on, it inoculates it with the virus . Semi-persistent viral transmission involves the virus entering the foregut of the insect . Those viruses that manage to pass through the gut into the haemolymph and then to the salivary glands are known as persistent . There are two sub-classes of persistent viruses: propagative and circulative . Propagative viruses are able to replicate in both the plant and the insect (and may have originally been insect viruses), whereas circulative cannot . Circulative viruses are protected inside aphids by the chaperone protein symbionin, produced by bacterial symbionts . Many plant viruses encode within their genome polypeptides with domains essential for transmission by insects . In non-persistent and semi-persistent viruses, these domains are in the coat protein and another protein known as the helper component . A bridging hypothesis has been proposed to explain how these proteins aid in insect - mediated viral transmission . The helper component will bind to the specific domain of the coat protein, and then the insect mouthparts--creating a bridge . In persistent propagative viruses, such as tomato spotted wilt virus (TSWV), there is often a lipid coat surrounding the proteins that is not seen in other classes of plant viruses . In the case of TSWV, 2 viral proteins are expressed in this lipid envelope . It has been proposed that the viruses bind via these proteins and are then taken into the insect cell by receptor - mediated endocytosis . </P>

Name one species of plant that is often attacked by the tobacco mosaic virus