Loss of the genetic diversity of some of the world's crops has accelerated in recent decades, with many crops becoming increasingly susceptible to diseases, pests, and environmental stresses. A global network of gene banks has therefore been established to provide plant breeders with the genetic resources necessary for developing more resistant crops that will enable farmers to maintain high yields. Most of these gene banks now store the germplasm of only the major crops such as cereals, potatoes, and grain legumes. Cultivated varieties of these crops are conserved as well as wild species that might otherwise become extinct. Tropical cash crops such as bananas and coconuts are also important food crops in many Third World countries, and more effort needs to be made to conserve the germplasm of these crops as well as of other important plants such as plantation crops, medicinal herbs, and fruit and timber trees.

Taxonomic investigations of the variation within Beta vulgaris sensu lato, including the use of numerical techniques, have led to the following conclusions: 1. The variation in morphology of the modern cultivated types of beet exhibit noda in a continuous variation pattern. 2. Microspeciation within wild forms of B. vulgaris is considered to have occurred in the centre of diversity, while in some areas hybridization within Section Vulgares has produced a complex taxonomic situation. 3. Because of the existence of this continuous variation, the classification within the section has been simplified. A new theory for the origin of cultivated beets is proposed.

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The morphology uC Rumex crispus L., R. obtusifolitis L. and putative hybrids was examined. There appeared to bo evidence of introgressive hybridization, particularly with backcrossing to R. crisptis. Seed germination in plants from a mixed population varied widely, but when plotted against the hybrid index for the parent planis ihc species could be partially separated in respect of germination behaviour. Some ofthe polymorphism previously described may have resulted from inclusion in the tests, togetlicr with pure parental types, ofplants of hybrid origin which had introgressed to such an extent that they were almost indistinguishable from the parent speeies. The implications of this arc discussed. Polymorptiisme des setmnces et germination IJ, Rdte de Vhybridation dans le polymorphisnie de ta germination deRumex crispus et de Rumex obtusifolius R^siun^. La morphologic de Rumex crispus L., de Rumex obtusifolius L. et de icurs hybrides supposes, a 6t^ examinee. L'^vidence d'une hybridation introgressive s'est manifestic, particulierement avec Ic ritrocroiaemcnt sur H. crispus. La germination des sentences dc plantes dans une population m^Iang^e varia considerablement mais, quand on les rapporta, quant a ieur degri d'hybridation, aux plantes parentalts, les espf-ces purent etre partiellement classics en fonction de leur comportement gcrminatif. Une pariie du polymorphisme prteddemment decrit pcut resulter de la presence dans les experiences, avec Ies types parentaux purs, dc plantes d'origine hybride provenant d'une introgression tellemcnt poussde qu'il est a peu pres impossible dc UTS distinguor des cspctes parentales. Les consequences de ces faits sont discutt^es. Samenpotymorptiismus und Keimung II. Die Bedeutung der Hybridisation beim Keimungsfwlymorphismus vonRumex crispus und Rumex obtusifolius Zusammeiifassung. Die Morphologie von Rumex crispus L., R. obtusifolius L. und mutmasslichcr Hybriden wurde untersucht. Es ergaben sich vor allcni bri Rlickkreuzung mit R. crispus Anhaltspunkte fur eine introgressive Hybridbation. Die Samenkeimung variierte bei Pfianzen von Mischpopulationen stark. Wurde sie fur die Elternpflanzen jedoch gegen den Hybridindcx aufgetragen, so konnten die Arten bezuglich ihres Keimvcrhaltens teilweise getrennl werden. Der fruhcr beschriebcne Polymorphism us mag teilweise daher geriihrt haben, dass bei den Tests ausscr den reincn Elterntypen auch von Hybriden abstammende Pflanzen verwendet wurden. die so starke Introgression aufwiesen, dass sie von den EI ternarten kaum noch zu unterscheiden waren. Die Bedeutung dessen wird diskutiert.

Summary. Achenes of Chenopodium album are polymorphic; four categories can be recognized visually, brown‐smooth, brown‐reticulate, black‐smooth and black‐reticulate. All four types may be borne by one plant. The polymorphic seeds differ in germination requirements, responding differently to chilling and nitrate application. Dormant seeds contain at least one extractable germination inhibitor. The inhibitor fails to act in the presence of added nitrate. Nitrate is replaceable by some other nitrogen compounds and by chlorate. Nitrate application stimulates germination of C. album in soil. The ecological significance of seed polymorphism and the relevance of polymorphism to the interpretation of germination tests are briefly discussed.