Lily hybrids vary in their ability to produce virus-free (VF) bulblets when grown from virus-infected scales in tissue culture. Asiatic hybrids typically produce a higher percentage of in vitro VF scale bulblets than do Lilium longiflorum cultivars. Three hypotheses concerning the cause of this variation are tested on five lily hybrids: an Asiatic hybrid, two L. longiflorum cultivars, an Oriental hybrid and L. candidum. The first hypothesis states that VF scale bulblets originate from wound tissue that is naturally low in virus concentration and blocks the passage of virus particles from one cell to the next. The second hypothesis says that scale-to-bulblet vascular connections, which serve as virus pathways, occur in hybrids showing high percentages of virus-infected scale bulblets, while connections are absent in those hybrids with low numbers of virus-infected bulblets. The third hypothesis concerns the virus concentration in the scale at the site of bulblet origin: bulblets of hybrids producing large numbers of VF bulblets originate from scale tissues low in virus concentration; bulblets of low percentage VF bulblet hybrids originate from scale tissues high in virus concentration. The first two hypotheses are not supported by the results of this study. First, lily bulblets do not originate from wound tissue. Second, scale-to-bulblet vascular connections consistently occur in 'Enchantment,' an Asiatic hybrid, and occasionally occur in L. candidum. Vascular connections are not detected in the low VF bulblet producers, L. longiflorum cultivars 'Ace' and 'Nellie White,' nor are they seen in the Oriental hybrid 'Stargazer.' Speculative support exists for the third hypothesis concerning uneven virus concentration in the scale. Distinct virus particles are observed with the electron microscope in the double virus-infected L. longiflorum cultivars and not in the other singly-infected lilies. The doubly-infected lilies produce a continuous layer of divided cells in the adaxial subepidermis of the scale where bulblets originate, whereas the singly-infected lilies produce cell division masses in the same area but only beneath forming bulblets. This study suggests that virus particles in L. longiflorum cultivars are more uniformly distributed than particles in the other lilies examined. This occurs not only at the site of bulblet origin but also throughout the scale mesophyll. Whether this is due to concurrent viral infection or to hybrid variation is unknown. / Graduation date: 1992