This thesis comprises two Parts and concerns the cytology of Lycopersicon and Solanum. Part I was a study of pachytene in Lycopersicon and began as an attempt to assess the usefulness of this stage as a source of information to aid classification. The principal findings of this thesis were as follows: 1. Chromosomes at pachytene were examined by the 'isolated chromosome technique', the precision of which was augmented by reference to probability theory. 2. In no case was it possible to identify all 12 chromosomes in any species examined but in L. esculentum sub sp humboldtii 9 chromosome types were elucidated. 3. Detailed interspecific comparisons of two distinctive chromosomes revealed that differences were of two kinds (a) difference in chromomere size and number, (b) differences in chromomere sequence presumably due to inversion. 4. Since such a small proportion of the genome was available for reliable interspecies comparisons pachytene morphology probably has little taxonomic value. 5. The occurrence of distinctive types once rather than twice in each nucleus provided morphological evidence for a basic number of 12 rather than 6. 6. Examination of diakinesis confirmed that at this stage the bulk of the chiasmata were confined to the achromatic region. This observation was however reinterpreted as evidence of terminalisation. 7. Evidence was obtained that chiasma formation occurred in both chromatic and achromatic regions. 8. The observation that chiasmata occurred at random throughout the tomato chromosome considered jointly with the existence of very small chromosome changes provided instances of 'cryptic structural differentiation’ and may account for a proportion at least of the genetic breakdowns observed in F\(_2\) populations of interspecific Lycopersicon and Solanum hybrids. Part II was a study of meiosis in polyploid species of Solanum and the principal findings were as follows: 1. Solanum juzepczukii may well have originated from the cross S. acaule x S. stenotomum. 2. S. curtilobum probably originated from a cross between S. juzepczukii and S. tuberosum. 3. During this study an attempt was made to define precisely types of chromosome configuration encountered
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