Plant viruses can develop high calibers of variation in their populations, but this is not always the case. The caliber of diversity in single plant infections varies dramatically with different viruses and different hosts. Recent studies on diversity in the DNA geminivurses betoken that they have variation levels that are commensurable to RNA viruses, in spite of their replication by the host DNA polymerase. Genetic bottlenecks occur during systemic infection of plant viruses and transmission events. Bottlenecks can have paramount effects on plant virus evolution due to genetic drift, and can ultimately result in isolation and evolution of incipient variants and speciation events. Plant viruses have been among the earliest viruses studied, with Tobacco mosaic virus  discovered by Martinus Beijerinck in 1898. Plant pathogenic viruses are parasitic nucleoproteins that vary in size, chemical composition, physical structure, and number of genomic units. They are transmitted in sundry ways, including by vectors vegetative propagation, pollen, seed, and by contact. Kineticism of viruses within a plant generally occurs via plasmodesmata for between-cell kineticism and via phloem for long distance convey. Symptoms associated with plant viruses include mosaic patterns, vein clearing, streaking, stunting, and hypo- or hyperplasia. However, plants can additionally maintain viruses without any conspicuous symptoms.