Various taxonomic methods are being used to clarify the systematics of Solanum section Solanum, the large and variable species group centring around the black nightshade, S. nigrum. To put the experimental work into perspective, a summary of former taxonomic treatments is given, together with an outline of the provisional taxonomic framework of the section, initially established from earlier classical studies, and supplemented by later experimental work. Experimentation with living populations has shown that Solanum taxonomy is complicated by several factors. In addition to the historical factors, phenotypic plasticity, genetic variation, the existence of a polyploid series, and the possibilities of interspecific hybridization accompanied by various pre‐ and post‐zygotic isolating mechanisms are dealt with. The various taxonomic categories attached to Solanum accessions during earlier classical studies derive support from the crossability behaviour of the taxa concerned. Morphological divergence is generally accompanied by genetical isolation which can manifest itself at any time between pollination and the maturation of the F2 progeny. Data from the hybridization studies, in which 14 diploid species, five tetraploid species, four hexaploid species and one octoploid taxon were used, are presented in detail.

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Population differentiation was studied in 32 accessions belonging to Solanum L. section Solanurn (Maurella); these accessions represented 20 species of four differing ploidy levels. A study of discordance demonstrated that the species exhibit considerable inherent discordant variation; it also provided an estimate of the relative variabilities of characters within these populations, and the consistency with which they differentiate populations.Hierarchic clusters, obtained using the singlelink cluster method, confirmed the relative importance of the different classes of characters used for differentiation within this section. Similarly, the hierarchic clusters generally supported earlier taxonomic decisions.The use of a non-hierarchic cluster method provided the most interesting data on differentiation within the section Solanum, particularly for genome relationships of the polyploid taxa. This clustering pattern fitted well with known genome relationships, while also suggesting putative ancestors for other allopolyploids; suggestions deriving retrospective support from the hierarchic dendrograms. The progenitors suggested by these two clustering methods supported data derived from orthodox taxonomy, comparative morphology, hybridisation studies, the artificial synthesis of polyploids and seed protein band patterns.

The nomenclatural changes which have become necessary for several species belonging to Solanum section Solanum since they were revised during the preparation of Flora Europaea are given and discussed. S. uillosum is now recognized as the correct name for S. luleum, and this has necessitated a new combination for the eglandular subspecies of' this taxon, namely, S. utllosum Miller subsp. puntceum (Kirschlegeri Edmonds. The two species were described simultaneously by Miller, and the precedence of the epithet uzllosum over the epithet luteum is fully discussed, together with the reasons for the selection ofpuntceum as the second subspecific name. The species previously referred to as S. melanocerasum is now known to be synonymous with S. scabrum, while S. chenopodioides becomes the correct name for S. sublobatum. Complete lists of synonyms are given for these three species, together with extensive notes o n the more controversial and difficult synonymy. Finally, reasons are given to justify the spelling ofS. sarrachoides with two r's.

Seed coat structure in Solanum L. section Solanum (Solanaceae). Seed coat structure in Solanum L. section Solanum was examined using SEM. The seed coat patterns observed were fairly uniform throughout the section, and thus of little diagnostic use. Moreover, all species are characterized by their seeds being covered by hairlike structures, which are bands of lignified thickening in the lateral (and basal) walls of the outer epidermal cells of the testa, and which only become obvious when the seeds are wetted. The development of the seed coat and its characteristic thickening was followed from the unfertilized ovule to the mature seed in vouchered material of Solanum nigrum L. since this species is the generic type, using sections of freshly fixed material.